Open Monday through Friday, 11:30 a.m. to 2:30 p.m.; Monday through Saturday, 6 to 10 p.m. AE, MC, V. Reservations required. Valet parking. Prices: At lunch, about $15 to $20 including tax and tip. At dinner, ,a la carte appetizers $3.50 to $7.25, entrees $12 to $17.50. Complete dinners $23.50 to $26.50.

Many years ago I came across an outstanding smorgasbord in Berlin, of all cities, at the Hilton hotel, of all places. A few years ago in Cairo I was repeatedly told that the best restaurant in town was the Hilton's. For years, in city after city, Hilton International has swum against the tide of disdain for restaurants in hotel chains.

So when Hilton International started building hotels in the United States under the name Vista International, its restaurants drew special interest--even more so when it chose as its theme that fashionable new cuisine: American.

The American Harvest Restaurant in New York's Vista has won mixed reviews, but it is an important restaurant. Its Washington branch, which opened in February, inherited the style and experience, a similar menu, and prices similarly high fixed-price dinners average $25 and main courses at dinner are about $16--though they are lower than those in New York. At first glance, Washington's American Harvest has all the assets of grand dining.

Vista's lobby is like a courtyard in a charming modern city. The hotel and dining rooms have windows overlooking the canopied bar (designed by Givenchy) and a grand old-style clock. Carpeted walkways lead to a wine bar and caf,e that looks outdoor and tropical. There are nice touches in each of the lounges and eating places: a tiny crock of liver mousse with your drinks in the Givenchy lounge, a chance to sample the wine bar's wares before you buy. In fact, I was impressed enough by the caf,e's breakfast menu to verify its promise that the orange, grapefruit and tomato juices are freshly squeezed.

But throughout these facilities there are slips that undo the excellent intentions. We asked at breakfast for hot water with fresh lemon juice. Orange, grapefruit, tomato--okay. But the waitress informed us, "We don't squeeze lemons."

The menu at American Harvest, too, implies more than it delivers. The dining room is attractive, carved into small rooms that could be your grandmother's home if you had a tasteful grandmother inclined to Chippendale, Lenox china and museum-quality early American folk art. Windows overlook the clock and the courtyard, and tables are lit by tall white candles in etched hurricane lamps. Early in the evening, a chamber group plays from a balcony across the courtyard.

The staff greet you effusively and bring a small hors d'oeuvre--perhaps raw vegetables and herbed sour cream, or shrimp butter with sesame crackers--and the large menu, both of which change monthly. Each month brings a new theme: citrus in February, broccoli in March, asparagus in April. April appetizers, for example, included asparagus and smoked salmon with potato pancake (delicious, and each element so good that it was the best of all the menu), asparagus vinaigrette with avocado (bright and refreshing), lobster and asparagus cocktail (watery lobster in a disappointing bland m,elange), asparagus bisque (tasting as if the kitchen had run out of asparagus before it got to the soup) and stir-fried asparagus with sesame seeds as a vegetable accompaniment.

The secret is out immediately: The best of American Harvest is in its accompaniments and garnishes. A tray of good homemade rolls might include Sally Lunn, corn sticks and potato rolls. Three vegetables are served family-style, and each one--perhaps cauliflower with browned buttered crumbs or carrots with chives--is impeccably cooked. With grilled meats comes a silver tray of seasoned butters--watercress, wine, sesame, horseradish, herb--in small slices to melt over your meat. Plates are garnished with fresh chervil or rosemary sprigs. After-dinner coffee can be brewed decaffeinated or double espresso. The wine list is an interesting collection of Americana and some good imports at restrained prices, and there are better than average wines by the glass.

But dinner is more than the accumulation of accessories, so along the way you are bound to make some sad discoveries. Maybe you've been misguided into starting with Crisfield Crabmeat in Young Cabbage Leaves. It's a damp and gummy roll of crabmeat shreds, oversalted and profusely peppered, wrapped in chewy cabbage leaves and sliced attractively on a bed of Russian dressing, garnished with a crayfish. It isn't as bad, though, as a sampler of four oyster preparations, each more dried out and bitter than the one before.

Maybe you've been clever enough to start with the crisp potato pancake and smoked salmon with asparagus, and then to go on to an overly sweet but aromatic spring onion soup. Take no more chances. The kitchen puts out a tender and thick tenderloin of beef and cooks it accurately. Topped with the seasoned butters it is a safe--in fact, very good--bet although it sits alone on its plate ungarnished and without the "boxed asparagus" the menu promised. Shrimp in Rapidan River Riesling on White Cabbage With Herb Butter is a delicious bed of julienned vegetables topped with shrimp of great size but little character. Shad and bacon was decent, though cooked a little too long, but it arrived without the promised roe and lacked the clean, clear taste of very fresh fish. Elsewhere on the menu, disasters lurk: charcoaled leg of lamb so overcooked that it was dry and anonymous gray meat; chicken breast coated with sesame seeds, cooked to tastelessness and topped with innocuous orange-ginger sauce; a $26 lobster that was ordered boiled but served grilled--grilled so long that its meat was chewy and its tomalley blackened.

Salad? I don't know. It was never brought, despite the fixed-price menu's promise. Finger bowls were, though; one day their water was hot enough to scald your fingers. The service has the air of a classroom--one server hissed for another to slow down--and ranges from suave to clumsy, though generally is as eager as if your dinner were the final exam. Yet nobody apologized or issued updates when our appetizers were nearly an hour in coming.

Dessert is a big production, the cart stocked with lattice- crusted pies and fudgy cakes, a wide array from the American repertoire. The apple pie is deep and tart and wrapped in a fairly flaky crust, as is strawberry-rhubarb pie. Fudge cake would have been wonderful except that its bottom layer was badly baked--flat and rubbery. Skip the papaya mousse and the creamy canned pineapple coconut log. The Lightning Layer Cake, though, was a revelation, a lemony and delicate American version of dacquoise, a nutted, lemony meringue and buttercream-layered pastry.

American Harvest is a beauty, its national pride endearing, its menu full of imagination. But the cooking swings from careful to amateurish. Its assets don't add up to a grand enough total. A fine vegetable does not make a $50 dinner.