From Capitol Hill to K Street, there's scarcely a block without one. Portable carts with mustard-colored umbrellas and Washington workers all queued up for hot dogs. Never mind today's emphasis on low-fat diets and better nutrition. Business is booming for the city's approximately 1,200 purveyors of the most all-American of all American foods.

Washingtonians love hot dogs.

In Alabama, they eat them slathered in coleslaw; in Kansas City with melted Swiss cheese, sauerkraut and Thousand Island dressing. But in Washington, pass the onions and shovel on the chili.

Altogether, Americans eat 50 million dogs a day or an average of 80 per person per year, according to the National Hot Dog and Sausage Council.

Even without the bun of about 100 fluffy calories, they are a nutritionist's nightmare: 13 grams of fat, 500 milligrams of salt and 145 calories. About 80 percent of the calories are fat, much of that unsaturated, according to the health advocates at the Center for Science in the Public Interest.

That may account for why sales nationwide are down about 10 percent. But you can't tell that by the street corners of the District.

Office workers out for fresh air love them, said Fran Altman, executive secretary for the Hot Dog Council. "You can walk around and carry it. It's a real portable food."

No one seems to know exactly how big an industry this American passion supports here, but city officials say most of the nearly 1,500 licensed food vendors in the District sell hot dogs. Many in the business are recent immigrants, newcomers from Egypt, Korea, Ethiopia, Nigeria and El Salvador. Others are people who have opted out of the traditional economic treadmill.

Some are old. Some are young. And most work for themselves, hoping to snare their piece of the American dream.

Take Leon Slade, for example. He is a 74-year-old transplanted South Carolinian who has been a hot dog vendor here for more than 30 years, elevating his fixings along the way to a culinary art form.

His creations are sloppy, overstuffed affairs with generous helpings of his own recipe for doctored-up canned chili, chunks of onion simmered in ketchup, and hunks of cheddar cheese loaded atop an all-beef dog or half smoke.

"He has got the best hot dogs around," said Mike Johnson, 20, a painter who frequently stops by Slade's stand outside the Petworth Employment Services Office on Kansas Avenue NW. "They just taste better than the other ones."

The onions are the difference, Slade said. "When {the customers} get the scent, they're like a dog scenting a person," he said, chopping a batch into one-inch chunks. "You'll have some of them come and say, 'Man, you cut those onions too thick.' I'll say, 'Chew 'em. That's what your teeth are for.' "

And don't talk to Slade, in his makeshift chef's hat and soiled apron, about the cheese dip that most other vendors spoon sparingly over their hot dogs. He uses the real thing.

"I cut it thick. They like this. They ought to. It cost $5.17," he huffed.

"When I was downtown, all the hot dogs tasted alike, the chili and all . . . . His taste like homemade," said Sabrina Gordon, 23, a clerk for the Occupational Safety and Health Administration, who said she can't get through a workday without a Slade hot dog.

His curbside manner, on the other hand, is a bit irregular, she notes. "Sometimes Leon can be very mean."

"The hot dogs were good," said Rick White, 27, a Howard University alumnus who remembers Slade from when he ran his stand outside the campus library. But "when you're serving people, you have to be friendly."

Slade admits he can be a tad gruff, but said he is not in the business to win a congeniality award.

"I have so many people come up to me and say I got a bad attitude. I say, 'You ain't eating attitude. You're eating hot dogs,' " he said. "You ain't eating what the man say. You're eating what he's fixing."

Hot dog fans are a loyal bunch. Some say they check out every vendor within a few city blocks before they choose; others say they go to the closest one to their office and then acquire a taste for the recipe offered.

Spyros Sgourdas, who has been selling hot dogs at 18th and N streets NW for nine years, has developed a customer base that is fiercely loyal. "He's a gentleman, Old World," they say.

"He's the only man to buy hot dogs from in Washington," said Theresa Simmons, a student at nearby Johns Hopkins School of Advanced International Studies.

"They come to me because I've got the lowest prices," said Spyros, 66, laughing and flashing a shy smile.

His prices are 10 to 25 cents below market -- 90 cents for a hot dog, $1 for a hot smoke or a half smoke -- and they haven't changed in years. He offers specials that people say can't be beat -- two hot dogs, chips and soda for $2.70.

But it's more than low prices. "They like me, the women like me," he said in his thick Greek accent. "Because I'm a nice man."

He stands by his cart, not inside as many vendors do, and greets each customer with a smile and a nod.

Then, with a grace learned during his days as a waiter at an upscale Washington restaurant, he plucks a hot dog from the steamy cavern of his steel cart, places it gently on a piece of white paper atop tinfoil, and then ladles on the mustard, ketchup, relish, raw onions, hot sauce, chili, and cooked onions in tomato sauce.

It's a rhythmic, precise dance that he repeats 100 to 200 times a day. He finishes with a twist of the foil, a shake of a brown bag, and a few extra napkins as an afterthought.

"He's different from the other vendors," said Thomas Hill, a senior manager at Ernst & Young, who has been a Spyros customer for nine years. "He's neat, clean, gentlemanly, always has a nice, kind word and nice smile."

But not everybody is happy with Spyros. A young, well-dressed man, obviously in a hurry, waited in line one recent noontime, watching Spyros slowly spreading on the mustard, adding the onions and taking his time. Frustrated and impatient, the man growled: "You'd never make it in New York City!"

That may be. But Spyros says he would never want to work in New York City. He has lived in the Washington area since 1968, when he left Germany. He later married Dimitra, his wife of 17 years. As a young man in Greece, he was a farmer in a village near Sparta.

He tried a lot of street corners before settling on his post at 18th and N streets NW. District law allows one vending cart on every 10 feet of sidewalk.

And no one has seriously challenged his domain, although Ed Angel, a public policy analyst at nearby Morgan Angel Associates, recalls one poignant lunchtime. A woman opened up a stand about 20 feet down the sidewalk and a line of nine or 10 customers at Spyros's stand just stood there and glared at her. "And I remember how mournful that poor woman was. She had no customers whatsoever," he said.

Spyros works five days a week, 12 months a year, unless it's raining or snowing. "It's too much cold. If it's snowing, ice, raining hard, no, I don't come." Otherwise, he's on the corner by 11 a.m., leaves by 4 or 5 p.m., and if it's windy, "I'm turning my cart this way, that way, so the umbrella doesn't fall."

It's not a perfect job. Business, he says, is down the past two years since new carryout places have opened around the corner and "people are eating more salads." And the price of hot dogs has gone up. "But if I changed prices for my customers, nobody would come here."

Eighteen years ago, Nadia Ahmed planned a career in hotel management. But then came a marriage proposal and a chance to leave her native Egypt for America.

Now 35, she operates a hot dog stand at 17th and L streets NW for 14 hours a day, five days a week -- regardless of the weather.

A strict Moslem, Ahmed keeps her head tightly covered in Islamic headdresses wrapped low over her forehead. The sleeves of her oxford blouse button at the wrists and her skirts stop about six inches from her feet, which are encased in thick gray socks bunched inside bright pink huaraches.

A grubby pink calico apron protects her clothes from spills. Ahmed's only nod to contemporary dress is an oversize pair of reflective aviator sunglasses. But she has modern ideas about American entrepre- neurship.

"I make this business five years now," she said in her halting English. "I work for myself . . . . I love it."

Still, it's a big change from Marriott, where she worked in housekeeping for seven years. She said she left that job because it involved contact with alcohol and pork, which is against her religion.

"No more working for hotels and restaurants. I have too much customers," she said, busily wiping her tiny counter. "It is hard work. It is not easy. Yes, I make a lot of money, but I must pay taxes. It is very hard."

Shortly after the lunch-hour rush, Ahmed's husband, who works nearby, relieves her for an hour, so she can nap or watch TV in a small apartment she rents nearby.

A mother of two children ages 15 and 17, she pays a nearby garage $20 a day to clean and store her cart overnight so she can go back to her house in the suburbs at night.

If the secret to the success of a hot dog stand is location and service, she has it down to a science.

"A hot dog is a hot dog," said Ron Whitehead, 22, an Eastman Kodak field engineer. "She talks to you. She makes you feel important. Sometimes she cheers me up when I'm having a bad day."

"I started coming because it's the closest to the building," said Barbara Meiherhoefer, a research associate at the Judicial Center, obviously in a hurry. "I don't know what I would do if I couldn't jump out of my building and come here."

"Can I have a hot dog?" piped the next person in line.

"With everything," said Ahmed.

"Aren't you something. How do you remember all that?"

Special correspondent Anne S. Crossman contributed to this report.

Purchased from city vendors per day:

Hot dogs...................20,500

Half smokes................13,000

Gallons of mustard........... 240

Pounds of relish............1,200

Pounds of onions............. 600

SOURCE: Estimates from distributors and vendors.