THE BOTTOM line on comfort this summer may well be shorts.

Not since the hotpants fashion of the spring of 1971 have shorts been so prevalent in Washington. It is no surprise with so many men and women wearing their shorts off the tennis courts and into the supermarket and other weekend shopping that shorts have been adopted as a cool, comfortable way to dress to leisure when it is hot.

With kids, shorts have reached uniform proportions: always the notched hem, elasticized waistline, gym-short variety, teamed with T-shirts or La-coste shirts, high white soccer socks with bright-colored strpes at the top and sneakers.

Hotpants first appeared under ankle-length and mid-caf coats in the Paris couture. But before one could say Yves Saint Laurent, they were adopted by young girls in Paris and worn here with skin-tight vinly boots and pantyhose, with those longer coats largely overlooked.

This time in Paris and New York, designers pushed shorts for warm weathr, many of them just-above-the-knee-highs, flat shoes, a shirt (and a blazer if it's cool).

But Washington women appear to have passed up these styles for the clean cut, tennis-short variety, teamed up with a T-shirt or silk shirt, espadriles or flats.

Eileen Rosenberger, a manager at Ann Taylor, says that that store sold now everyone wants the short-short variety. Among her best sellers are those in terry cloth "which are very sexy," she says. "Last year in Washington a lot of men and women wore shorts, but in a sloppy way. This year it is a fashion look and they are buying them with a matching top."

At Saks Fifth Avenue, too, short-shorts are far outselling the longer ones. "It makes great sense as an alternative to sundresses," said Vashti Mays, the store's fashion coordinator. Garfinckel's most popular styles have drawstring waistlines. "It's become the thing to wear on Saturday afternoons," said Janet Wallach at Garfinckel's.

Louis Lehman, president of Sunny's Surplus, expects to sell over 28,000 paris of gym shorts at $2.49 in the Washington area in four months. He sells more shorts to women than men, he said, with probably many of the small sizes being bought for children. According the Lehman, his customers are wearing them for jogging and bicycling, "and even for dates."