One of the best vacations I ever had was a 10-day trip I took through Ireland ... alone.

And one of the worst vacations I ever had was the time I split off from friends in Athens to explore one of the Greek islands, came down with a debilitating case of the flu ... and had to fend for myself.

Adventure or nightmare? Traveling solo can be an unfettered, impulsive escapade, with no responsibilities and nobody to answer to or for. Or it can be a lonesome journey and, for some, a downright scary proposition.

The "Let's Go" guidebook series actually advocates going it alone, suggesting that independent travel offers more pleasures than pitfalls.

"If freedom and spontaneity are your first priorities, consider seriously traveling alone," the guides, published by Harvard Student Agencies Inc., advise. "This can result in a sense of isolation but does force contacts with the locals. Your preferences will be given full expression as you take off on a whim, backtrack or stay somewhere longer than planned."

Friends, according to the guides, "are a great source of energy and comfort, sharing in food and lodging costs and providing companionable security. But steady traveling with one person or group may insulate you from the local culture. You may want to split up occasionally to enrich your personal experience and gain a new appreciation of your partners."

Easier said than done.

"I know some clients who wouldn't want to fly alone let alone travel alone," says Giulia Adelfio, assistant manager of the Bethesda Travel Center. She says singles travel "depends on the individual" and isn't for everyone.

"There are some people who go and rent a hotel room alone in the Florida Keys and read and lie on the beach for a week and have a great time," she says. "And there are others who couldn't spend even one day alone without going crazy."

Travel agents and some tour companies can put together itineraries specifically tailored to single travelers, those who want to be alone and those who just don't have a travel companion. Singles can also arrange trips that leave them on their own as much as they wish but bring them in contact with other travelers, including those also traveling alone.

Of course, going solo is usually more expensive than traveling with a partner or group. Hotel rooms cost more and some tour groups are apt to levy a special "single supplement" charge on private accommodations. Also, traveling alone may pose particular difficulties for women in some countries.

Singleworld (1-800-223-6490), a Manhattan tour organization that caters exclusively to single travelers, offers money-saving share rates on all its cruises and tours. For those traveling alone on cruises, it will match roommates -- smoking preference and age are a consideration -- to avoid the high cost of a single cabin. Christmas/New Year's holiday cruises of three, four and seven days have been increasingly popular.

Cosmos Tourama (1-800-222-0090), which gets a third of its tour business from clients who are single or traveling alone, says it will "guarantee shares" and match up compatible singles in twin-bedded rooms. And if it can't find a roommate for a client who has booked on a guaranteed share basis, Cosmos promises to absorb the single supplement and put travelers in a single room at no extra charge.

For women traveling on their own, there is a new book that might prove useful. In "Going Alone: The Woman's Guide to Travel Know-How," published by Hippocrene Books Inc., author Carole Chester takes you from Australia to Zimbabwe and alerts you to any cultural or "male attitude" problems that might affect a woman's travel experience in particular countries.

Some of Chester's travel tips are standard guidebook stuff -- where to stay, getting around, medical emergencies and useful addresses. But her advice and observations on safety and security, eating and drinking solo and where to go and not go at night alone speak to the concerns of women travelers.

Still, the success of independent travel often has a lot to do with where you go and what you expect.

When I did my solo tour of Ireland, for instance, I had never traveled alone before and didn't set out to go it alone on this trip. I was in London with a boyfriend and another couple, and they, having no Irish roots whatsoever, wanted to go to Spain. I was determined at long last to see the Old Sod.

My friends told me I wouldn't like traveling alone, that it was dangerous and lonely and that, anyway, I'd probably overromanticized the place. I told them I'd meet them back in London.

Once in Ireland, I found I was so happy to be there and so caught up in the place of my ancestors that traveling alone just added to the experience. Not that there weren't a few problems: Fresh off the boat, I took a train, got off at the wrong stop and ended up in rural nowhere, in the rain. I had to hitchhike to the nearest town, another first for me. Also, I didn't drink beer and felt distinctly out of place at pubs. Even worse, I was an Episcopalian who admired Bernadette Devlin, and I can't begin to tell you the kind of political trouble that got me into, especially in Belfast.

Yet what I remember most from the trip is that I explored the country according to my own pace and curiosity, and that as an Irish American traveling alone, I was virtually adopted by the wonderful people in that country. Looking back, I realize that much of my itinerary had a lot to do with Yeats, Thomas Moore, Joyce, O'Casey and O'Flaherty, the Irish poets and writers, in that I kept turning up in places where they had been born, lived in or written about. A travel companion might not have understood.

So 10 days alone in Ireland wasn't enough, and I've returned there many times since.

By contrast, I once spent a half day in Seoul alone and suffered such culture shock that I retreated to my hotel room rather than explore this first stop on a trip to Asia. It wasn't until I met up with some Americans in Hong Kong that I felt confident enough to play tourist.

When traveling alone, there are some obvious spots to avoid if you don't want to feel out of place. Avoid couples and family resorts if you're not part of a couple or a family, and don't take dinner by yourself at one of those romantic inns.

By the same token, there are some equally obvious places where single travelers can go and be reasonably sure of having a good time. Big cities in just about any country offer lots to do. There are theaters, parks, concerts, museums and galleries and you can amble around according to your own whim. Cruises offer plenty of activities, plus a shared dining-room table for conversation. Or if you like to ski and want to improve your skills, book a week at a lodge, join a class and meet some like-minded skiing companions.

There are a number of organizations that can help organize single travel, among them:

Club Med. Todd Bardes, of Omega World Travel here, says Club Med resorts are great for single travelers. They emphasize socializing -- meals, in fact, are seated family-style -- and he says he has never had a client who didn't have an enjoyable time. For more information, call Club Med, 1-800-528-3100.

Womanship. If you're a woman who has never felt comfortable handling a sailboat or single- or twin-engine trawler, contact this Annapolis-based program, which offers lessons in the Chesapeake Bay, Florida, the Virgin Islands, New England and the Pacific Northwest. Women who have never operated a boat and women with considerable experience can attend sailing or powerboat clinics geared to building skill and confidence. For more information, call 1-301-267-6661.

Guides For All Seasons. This Carnelian Bay, Calif.-based tour operator specializes in small group adventure travel to such destinations as Nepal, Tibet, the Tongan islands and Thailand. Hike the Japanese Alps or join a Mount Everest expedition. For information, call 1-800-457-4574.

Carnival Cruise Lines (1-305-599-266) is one of the lines that lets individuals book on a single basis and comes highly recommended by travel agents. Or phone a travel agency that specializes in cruises.

In addition, there are tours organized around a particular activity, such as hiking, skiing or biking. Among them:

The Sierra Club puts together inexpensive hiking trips throughout the United States. For information, call its Outings Department in San Francisco, 1-415-776-2211.

Open Roads Bicycle Tours in Haymarket, Va., organizes local inn-to-inn bicycling groups. Call 1-703-754-4152.

LONDON DETAILS: Visitors in search of detailed tourist information in London can visit the new British Travel Centre at 12 Regent St. recently opened by the British Tourist Authority. Just off busy Piccadilly Circus in the heart of the city, it is described as a "travel super-store."

It fulfils the standard task of a tourist office to provide sightseeing and lodging information. But visitors also can book hotel rooms (or other types of accommodations), not only in London but in the rest of England, Scotland, Wales and Northern Ireland.

The new facility also sells rail tickets, sightseeing trips, theater tickets and guidebooks to Great Britain. There is a currency exchange desk. The office is open Monday through Saturday from 9 a.m. to 6:30 p.m. and Sunday from 10 a.m. to 4 p.m.

-- James T. Yenckel