Let's say you're an intermediate skier in good condition. You've skied moguls, big and small. You've schussed some of the famous trails in the Rockies and the Green Mountains. Now you're getting a little bored with it all.

What do you do?

You do what I did -- you go to Courmayeur-Mont Blanc, in the Italian Alps, and ski down the side of Mont Blanc, the highest mountain in the Alps. The run over glaciers is called the Valle'e Blanche, and since it's more than 12 miles long, it seems to go on forever.

"All you need is a guide," said Leo Garin, a Courmayeur businessman who is one of the region's leading mountaineering experts. I had told him I was thinking of skiing the Valle'e Blanche.

Could I believe him? Already Garin, whose private library about Mont Blanc numbers 1,400 different volumes, had told me the story of two brothers who ventured down the Valle'e Blanche a few years ago. One brother fell into one of the crevasses that dot the Valle'e Blanche like holes in Swiss cheese. The other brother tossed his mountaineer's rope into the crevasse and began hauling it back up.

When he finished pulling the rope back up, the skier on the other end was not his brother, but another skier who had fallen into the crevasse earlier. The moral of the story is clear: Although the Valle'e Blanche is relatively tame, it is full of hidden dangers. Do not ski it without a guide.

One of the charms of the Valle'e Blanche is that the longest run and the best route for intermediate skiers starts in the quiet village of Courmayeur in Italy and ends in the bustling small French city of Chamonix. Typically, a guide will take six or seven skiers down the run in a day-long excursion for about $25 a head. Add that to the $14 per-person cost of the lifts to the actual starting point of the run at Punta Helbronner -- at the Italian-French border on Mont Blanc -- and you have one of the great bargains in international skiing.

The village of Courmayeur is in the semiautonomous Valle d'Aosta, tucked between France and Switzerland in the far northwest corner of Italy. Today the village remains appropriately schizoid -- an Italian town with a heavy French accent. Both Italian and French are taught in the local schools.

On the night before my trip, Courmayeur's relatively low elevation of about 4,000 feet does not interfere with my sleep. And that's no small thing, for I, like many other skiers, often toss and turn all night at high altitudes. The next morning, the trip up Monte Bianco, as it is occasionally called on the Italian side of the mountain, starts in the town and I soon get my first hint that I'm in for a run of heroic dimensions. I base this assumption on the simple premise that what goes up must come down: What goes up long must come down long.

Up and up goes the network of cable cars. An hour and a half after leaving my hotel, our third and final cable car leaves our group of six at Punta Helbronner. Someone announces that this is the highest international border crossing in Europe, but it is the chilling cold and the raw wind that catch our attention now. It is a rude wake-up call for those of us who have been daydreaming and watching the breathtaking scenery on the way up. Now I wonder whether I have made a mistake. Am I in over my head?

Famous international border or not, the Punta Helbronner is windswept and the tiny boardwalk with a sheer drop off one side is barely wide enough for skiers to walk single file. The peaks of Mont Blanc -- the mighty massif is really a cluster of peaks -- rise around us to a height of 15,781 feet, just to intimidate us some more. Stowe and Aspen were never like this.

The run was billed as a cakewalk, but now the wrath of the wind and the jagged granite peaks bring hesitation. The local promotional hype proclaims Mont Blanc the "roof of Europe," and no doubt it is, but I know also that poet Percy Bysshe Shelley dubbed it "a city of death." These Mont Blanc peaks claim a dozen people a year and, in a particularly bad year like 1979, more than 40.

Still, no one hesitates. Women and children go forward. I press on.

We all have standard alpine skis and boots, and we snap into our bindings. Our guide, Giorgio, has short, fat, flexible mountaineering skis. Before long we will learn why he uses them.

A few quick turns and we're out of the wind and into the sun. Mont Blanc is in the southern, so-called sunny side of the Alps, and generally the weather there is comfortable for skiing from mid-January on, although of course the occasional storm can embarrass any weather forecaster around here. Last year there was good skiing on the Valle'e Blanche from the Italian side through April.

We are now skiing the Grand Flambeau, a moderately sloping glacier that typically opens up for summer skiing in June after the long traversata down the Valle'e Blanche is no longer possible on skis.

Immediately, we find that the snow is tricky -- a few inches of new powder over a crust that breaks occasionally. No wonder: There are no snow cats grooming the pistas at night here. I curse the long and stiff 205-centimeter skis I rented in Courmayeur that handled so well on Courmayeur's smooth and manicured slopes. I would have been better off today with short, soft skis.

In the distance we see two other groups of skiers making their way across the Flambeau toward the Valle'e Blanche. It's impossible to see or hear a lift or cable car, and this glacier skiing above the timberline bears little resemblance to the trail skiing the Americans and Canadians in our party have grown up on. We're on our own now, to be led down by our guide.

Giorgio is a representative of what is called "the profession of guide" in the Italian Alps. He has passed through a vigorous gantlet that requires him to be a strong skier, capable of performing arduous rescue work and proficient in giving first aid in the mountains. After taking a six-week course, apprentice guides must serve a three-year period before they are fully certified. It's a very serious business, and when you're in the mountains with them, you're glad it is.

Since the late 1700s, the guides here have operated almost exclusively in the spring, summer and fall months, but in the past few years ski mountaineering has enjoyed a mini-boom, opening up a new opportunity for the guides. Many guides lead skiers in week-long ski treks from hut to hut on the great Haute Route and its tributaries that snake along the top of the Alps between Courmayeur, Chamonix and Zermatt, Switzerland. Ten years ago, Giorgio likely would have spent the winter months as a wood carver or ski instructor.

Now we leave the Flambeau behind. It's been a perfect warm-up. We drop down the backside of a crest and suddenly we hit the first se'racs and crevasses of the Valle'e Blanche. It is steep here, and soon there is a small queue as skiers from the French side funnel into the Valle'e Blanche. (More difficult than the run from the Italian peaks, the way down above us from the Aiguille de Midi should be attempted only by expert skiers in top shape.)

Giorgio shakes his head at the hot dogs from the French side, some of whom have ventured forth without a guide in a display of mountain machismo. Daredevils have never had immunity from disaster in the mountains: The story is often told here of one of the area's best skiers, Louis Lachenal, who violated one of the fundamental commandments of the guides when he set out on a trip without a companion. When he didn't return at the expected hour, mountain rescuers set out after him. They found his ski tracks and followed them carefully until they disappeared in a crevasse.

Since there's no way of telling precisely how deep the various crevasses are, it's advisable to give them all a wide berth. Giorgio moves our party carefully over snow bridges and around crevasses. The biggest problem -- even for the experienced guides -- is that the icy terrain changes. New snow can easily cover over a crevasse. This is a place where people obey Giorgio's orders. No one wanders off on his own.

Suddenly, a woman in our party falls and begins sliding slowly toward an open crater. Everything seems to happen in slow motion. One of the skiers stretches his pole out for her to grab, but she is momentarily paralyzed and continues slipping. In a flash Giorgio scampers to the edge of the crater and blocks the woman's slide. Now we see the value of Giorgio's short flexible mountaineering skis.

The woman thinks she might have stopped in time anyway, but who can be sure? I try to look down the crevasse, but I can't tell whether it's 10 feet deep or a cleavage of China Syndrome proportions. I really don't want to know.

Soon we've left the steep, icy area behind and we're on La Mer de Gla~ce, the huge glacier that hooks up with the Valle'e Blanche and flows down toward Chamonix. La Mer de Gla~ce is a wide moonscape, nearly flat, and eerily quiet. Only the occasional crunch of ski poles digging into the snow reminds me that I'm not alone.

After two or three miles of gliding along la Mer de Gla~ce, we take a brief lunch break at a small outcrop of rocks. The February sun is warm, and we're comfortable sitting on our small knapsacks. This being my first trip to the Mont Blanc area, I was surprised to find such perfect skiing weather in the dead of winter.

We're on the last leg now and impatient to get down. There's one last area of se'racs and crevasses and, finally, one hair-raisingly steep but thankfully short pista that many in our party traverse rather than take straight on. The out-of-shape intermediate skiers are visibly tired and shaky on their skis now, but we all know we're close to the end when we ski onto one of Chamonix' groomed trails.

The return to reality is swift. The slopes here are crowded with flying skiers, the trails rutted, icy and worn. It's pretty much the kind of skiing we've known throughout our skiing lives in the States. The Valle'e Blanche will spoil us forever.

Once in Chamonix, we opt to return to Courmayeur rather than tarry in the small French city. We hop a bus back through the Mont Blanc Tunnel for $5 and we're back in Courmayeur in 30 minutes. It seems silly, but French officials check our passports carefully to make sure we have the visas that France requires for border crossings, even for our little jaunt into Mont Blanc.

The French have discovered Courmayeur, and the "in" thing to do now is to zip through the tunnel for dinner at the incomparable Maison de Filippo, where a 35-course meal (yes, 35) has earned the restaurant the moniker "Chalet of Gluttony." The bustling Valdostan tavern specializes in regional food like camoscio (chamois meat) and Valdostan boiled dinner of ham hock, cabbage and potatoes. While some visitors come to Courmayeur just to eat, most come primarily to ski.

And why not? With four cable cars, six chair lifts and about 20 other assorted lifts, Courmayeur itself is a full-blown ski resort offering more than 60 miles of ski slopes. Nearly 25 miles of groomed cross-country skiing trails are laid out in a remote area of stunning beauty that takes skiers over hill and dale, by chapels and country inns, all framed by the Mont Blanc massif. There are a skating rink with artificial ice and indoor swimming pools. And there's a wide range of restaurants, many of them located in the more than 50 hotels in the town that serve up fare robust enough to buck up anyone for a day in the mountains. And, of course, like the other major ski resorts of Europe, one can eat like a king at the restaurants on the ski slopes themselves. La Grolla at the Pendeint ski lift serves such good and hearty fare that many skiers who break for lunch there forget skiing for the rest of the day.

So, what's missing?

Glitz, that's what. The glitterati seek out the European ski resorts with wall-to-wall fur coats and leather pants. Fur coats aren't forbidden in Courmayeur, of course, and there are a couple of discos that stay open until 4 a.m., but life here is quieter and people aren't on the make so much. They come to ski and trek and relax, rather than posture and party. Courmayeur is a favorite winter vacation spot for families. Many serious skiers and mountaineers come here. For instance, the heavyset man with the white hair you see bombing down the Gigante may well be Pope John Paul II, who likes to ski in Courmayeur.

Courmayeur is many things, but above all it's still a small Italian village in the Alps, with a permanent population of just 3,000. Mountaineering sets the tone of the place. One of the main attractions in the town is the Museo Alpino, with its displays honoring the guides. In the latter part of the 19th century, the local guides participated in pioneering expeditions organized by the Duke of Abruzzi, who set a world north latitude record near the North Pole in 1890. Today, there is a steady flow of mountaineering trips and expeditions setting off from Courmayeur to mountains everywhere -- Nepal, Tierra del Fuego, Iceland, Yosemite.

With Mont Blanc in the background, such mountaineering trips elsewhere seem like taking the proverbial coals away from Newcastle. Even the most experienced skiers, the skiers who have been everywhere, the skiers who have done it all -- from skiing the headwall of Tuckerman Ravine on Mount Washington, in New Hampshire, to helicopter skiing in the deep powder of the Canadian Rockies -- will rank the Valle'e Blanche high on their list of all-time ski thrills.

Late at night, when the wine and beer flow faster and faster in the hotels of Courmayeur and the tall tales get taller and taller, sometimes as tall as the mighty Mont Blanc itself, even the intermediate skier can keep up with the very best of them when his turn comes to tell of his trip down the Valle'e Blanche. Because everyone who makes the famous traversata has a great tale to tell.

W. David Gardner is a writer and skier who lives in New York City. WAYS & MEANS

GETTING THERE: You can reach Courmayeur by car or train from Milan, Italy; Geneva, Switzerland; or Lyon, France. But prices in this area can be jolting. For instance, a short cab ride around Courmayeur can run $25. So package tours offer the best bargain for getting to Courmayeur. There are a number of seven-day tours available from Washington and New York that range from $825 per person (based on double occupancy) for modest but comfortable accommodations at pensiones to $1,450 for luxury hotels. The cost includes air fare, transfers from Milan to Courmayeur, a room, some meals, ski-lift passes and discounts on ski rentals and ski instruction. For the Valle'e Blanche trip, a travel visa to France is required.

Alitalia Airlines (800-223-5730) offers various packages to Courmayeur, as does Central Holiday Tours (206 Central Ave., Jersey City, N.J. 07307, 800-526-6045). INFORMATION: For information on the Valle'e Blanche trips, contact the Society of the Guides, Maison des Guides, 11013 Courmayeur (Aosta), Italy, phone 011-39-165-842-064.

For more information, contact:

Courmayeur Tourist Information Office, Azienda Autonoma de Soggiorno, 11013 Courmayeur (Aosta), Italy, phone 011-39-165-842-060.

Italian Government Travel Office, 630 Fifth Ave., 15th floor, New York, N.Y. 10111, (212) 245-4822. -- W. David Gardner