Somehow, nothing ever seems quite so new as something old rediscovered. American Ballroom Theater, which opened a sold-out three-performance engagement at the Kennedy Center's Terrace Theater last night, dances the way people used to dance on a Saturday night, but in a completely contemporary way. Half the audience must have watched with nostalgia; those of us who grew up in the post-twist era could feel only envy.

As its title implies, the company performs once-popular social dances -- the quickstep, lindy hop and cha-cha, among others. Some selections are pure exhibition numbers; others are embellishments to or abstractions of a dance. The numbers, grouped in four "acts" (The Rainbow Room, La Argentina, The Latin Quarter and Sheer Romance), flow one into another with effortless grace. The drama -- and there is plenty of it -- is in the romance inherent in social dancing and the vitality and polish of the performers.

Each of the four couples is a champion of international ballroom dancing competitions; each has its own distinctive style and personality. Company directors Pierre Dulaine and Yvonne Marceau are a chic adagio team; Wilson Barrera and Margaret Burns, the Spanish specialists; John and Cathi Nymchek are quick and spunky; and Gary and Lori Pierce combine elegance with a fresh, boy-and-girl-next-door athleticism.

John Roudis' choreography, whether for couple, trio or full company, never intrudes on the dancing, and this is part of the company's charm. They're not pushing Art, they're dancing. They look totally natural -- superhuman, but natural -- and watching the show is like spying on a private party of medalists gliding and hopping, changing tempo and rhythm incessantly, getting a bit giddy, and most of all, having fun.

American Ballroom Theater presents about as enjoyable an evening's entertainment as can be imagined. Those who have missed this engagement may be consoled by the fact that the Kennedy Center has booked the company for a rerun this coming February.