In a seminar once at the University of Virginia, William Faulkner was asked why so-and-so had a hat on in one of his stories. "To keep his head warm," Faulkner said.

Ike wore a Hamburg to his inaugural. Kennedy like to put on fedoras. Hedda Hooper made hats part of her head. John Wayne put on a green beret for a war movie. That happened just as "hard hat" was taking on sociological import. In the '60s, hair itself became the essential American headgear.

So here, in the dining room of a modest brick Northwest house, is the definitive Washington hatman. David Fleischer is a doctor, a gastroenterologist, and an assistant professor of medicine at George Washington University medical school. He is a tall, angular, soft-talking, serious-minded 33-year-old Kentuckian in a blue oxford shirt and a polka-dot tie and something called Dr. Murray's Space Shoes.

He isn't wearing a hat.

He just collects them. Any kind of hat-bowlers, derbies, boaters. Berets, fedoras, fezzes, lam-o'-shanters, sombreros, sunbonnets. Football helmets sparring helmets, pith helmets. Panamas, pork-pies, Stetsons, British naval hats, Spanish bullfighters hats, Philippine gourd hats. Hard hats, top hats, dunce hats. Aviator caps, baseball caps, stocking caps. Even, as he will proudly show you, Old Yellow Cab hats.

In fact, that's how all this started. David Fleischer once drove a Yellow Cab. That was in Louisville, in 1966, after college at Washington and Lee and just before medical school at Vanderbilt. The son of a middle-class hardware store owner, Fleischer wondered if there wasn't a novel buried in his hack-driving experiences. "Old 408 Yellow Stanby" still isn't finished, but Fleischer did get himself a heck of a hat - No. 1 in his collection today. It's a terrific hat: shiny black brim; snap-on yellow plastic band complete with hole for your pencil, a motto up top on the canvas peak that proclaims, "Every Driver an Escort."

"At the end of the summer they told me I could keep it or turn it back in for my $2 deposit," Fleischer says.

He kept the hat - and entered the fraternity of world class hat collectors. Now he's got a hat from Cassius Clay's Louisville gym and has been exhibited in two Florida cities.

Not that it proceeded in lock-step march from that Yellow Cab hat. A week or so after he kept his taxi hat, a spiked helmet from a flea market wandered into Fleischer's life. Then came one of his father's discarded Shriner hats. The next summer Fleischer picked up four or five headpieces at Portobello Road in London.

Girlfriends stated saving them for him: One almost consented to bed down with an English bobby in exchanged for his hat; another brought a French butcher's cap from Paris - and stunk up the plane coming home.

By his last year in medical school, Fleischer owned enough exotic headgear to host the first Annual Hat party, a moveable feast that has since been celebrated wherever his medical work has taken him - in Cleveland, Los Angeles, Washington, and Wellington, New Zealand. "That was our only international conclave," the hatman sights.

The single requirement for admission to a Fleischer hat party is wearing a hat. All manner of people wind up at these parties.People have been known to travel cross-country. Last May, at the ninth annual, Dr. Hyman Zimmerman, a nationally known physician, showed. The top prize winners came wearing fruit ballons all over their bodies. They were supposed to be a Fruit of the Loom underwear ad. Now there's a Fruit of the Loom commercial on television based on the same idea.

Larry Ross, a Washington lawyer, attended his first hat party last year. He dressed as a Southern politician - white suit and white panama. A lot of serious Washington professional people turn up, Ross says. "They come out. It's not quite your stiff Washington dinner party."

David Fleischer isn't the only person in America who collects hats. "Miss Helen" in the San Francisco area is a retired milliner who has been saving them for nearly half a century. Another milliner, Ben Greenfield, in Chicago has been donating his collection to the Chicago Historical Society; so far he has given over 400.

The Oakland Museum in Oakland, Calif., recently held a hat exhibit called "Hats Off." Says Inez Brooks-Myers, associate curator, "It's my experience if it's made, somebody's going to collect it." There are no hat societies or annual conventions she knows of, as there are with beer cans and campaign buttons. "It's mostly an individual passion." She had not seen Fleischer's hats, but guesses anything over 100 pieces would be considered a substantial collection. Fleischer has ovrer 100 pieces would be considered a substantial collection. Fleischer has over 150 at the moment, and discarded or given away many more. "It's a perishable art, after all," says Brooks-Myers.

"I don't think collection of things are hobbies," says Fleishcer. "Hobbies are things like gardening or restoring old cars. What hats do is form a kind of diary of your life. They're like old songs on a radio."

He says this and then goes to the living room. He burrows into a huge box, unwraps something from tissue paper, comes up a second later wearing a marching band hat. The thing has a gaudy orange plume floating from it. "Each one has a story," he says launching into a story about how he wrangled this one from the prop department at Disneyland. He is standing in the middle of the room very serious - a physician gone bonkers for hats.

"Now David," says Karen Fleischer, the doctor's wife, watching from the doorway. "Tell the truth. You added the feather."

"That's true," says Fleischer. He looks disconsolate.

What collecting hats may finally mean to David Fleischer is a guarantee of individuality. That and escape from the sober world of X-rays and barium enemas. "Doctors are supposed to be serious people. People come to them because they have something on their minds. I mean, Karen will tell you right off when Im in my doctor voice. She catches me at it all the time. When I have a hat on at one of my hats parties, I really come out.

"I remember a prof in med school telling us how fulfilling it can be to take a small sub-branch of medicine and get to know it better than almost anybody else in the world. Well, there are hundreds of doctors in Washington and there are dozens of gastroenterologists, but there's probably only one who knows all about hats. That's probably why I do it."

One of the sadder things about hats, David Fleischer notes, is their steady fall from grace in this century. Once, you weren't considered well dressed without something on your head. It was socially incorrect to be uncovered. There were hats for riding and there were hats for motoring and there were probably hats for sitting in the bathtub.

Now people wear hats primarily in connection with their work (steel workers, coal miners) or out of an urge to stylize themselves (Superfly, cowboy rock bands). Ironically, as social headgear has wanted, functional hat-wearing has waxed. Football, for instance, has gone from a game of unprotectec, head-busting to a game where the participants wore leather caps to a game where the players now wear hard plastic helmets that some people say ought to be registered as deadly weapons. Things have a way of getting back to square one.

Hat collecting is "like a sine wave for me," David Fleischer says. Right now, the pasttime is in an "up phase." He has other outlets: offbeat verse and G-rated home movies with elaborate subtitles.

"My interest in hats will drag some this winter. But by spring I'll be thinking og the next hat parties is that the hats get a chance to come out of their boxes where they have to live all year."

The hatsman stops. "Somebay I'd like to think I'll have a house with a special David Fleischer Memorial Hat Room. I'll be able to visit them anytime I want."

Puzzled looks, then a grin, not quite sheepish. He is considering revealing something about himself. "If you really want to know, I don't even like hats that much. I never wear them."