Five years later, Mary Knipper still remembers the pain of reading the letter from her son, the dentist, announcing he was hanging up his drill. "I was devastated," Mrs. Knipper says. "I thought, 'Why, after I worked all those years to put him through school, is he doing this?' "

A visit to the No Quitters health club in Fort Worth, Texas, explains. Inside, 47-year-old Jeff Knipper can be found advising clients on proper biceps-bolstering technique. As a fitness trainer, he earns a third of what he did as a dentist, and nobody calls him Doctor. But no patient in his 14 years of dentistry ever appreciated his attention the way his fitness clients do.

"He's totally focused on you when he's working with you -- and he has an incredible body," says Susan Croft, a client.

Transformed at the Gym

A strange thing is happening at the gym. Some professionals entering the locker room as lawyers, accountants, sales managers and such are emerging ultimately as personal-fitness trainers. They go to the gym in search of a physical-fitness workout and instead find a calling -- which is a problem only in that they already have a profession. "There probably aren't many other members of my law-school class who wound up working as fitness trainers," says Sandy Gresko, 35, a Notre Dame-trained lawyer who practiced law for nine years before becoming a trainer.

Many dentists, accountants and lawyers can't help noticing that their clients dread any encounter with them. That helps explain why "dentists have such a high suicide rate," says Mr. Knipper, whose mother now supports his career move.

But in gyms, fitness trainers are celebrities. They know everybody, and they usually look great. Their devoted clients feel grateful for the undivided attention -- even if it comes at the cost of $25 to $100 an hour. When the relationship brings results, that gratitude can evolve into worship. "You wouldn't believe the warm and fuzzies I get from helping a gal into a pair of jeans she never thought she could fit into," says Bob Fields, a 42-year-old Indianapolis fitness trainer.

A Self-Esteem Thing

Think computer nerds are hip? So did Mr. Fields. He worked at computer stores during his school years, got a degree in electrical engineering and became a software programmer. But it wasn't until he refashioned himself as "Trainer Bob" and became the official trainer for the Indianapolis Colts cheerleaders that he could say, "Every morning I wake up, I think: It's good to be me."

The number of certified fitness trainers is estimated to have doubled in the 1990s, to about 100,000 nationally, to serve the growing number of professionals eager to pay somebody to show them how to lift weights, climb stair machines, lower blood pressure and lose weight. In the service economy, fitness trainers are hot for the same reason nannies, pet sitters and personal shoppers are popular: Many professionals have more discretionary income than time.

But there's a difference: How many business executives envy their housekeepers? At age 40, Mike Foreman hired a fitness trainer to help him combat the stress and fatty diet that came with his job as general manager of a popular new Dallas restaurant. But Mr. Foreman quickly had a startling revelation: "I liked his job better than mine." Mr. Foreman is now a trainer.

Still, isn't it painful to turn your back on years of college for a job at the gym? "Why throw good time after bad?" replies Kurt Chacon, a 40-year-old Dallas trainer with a law degree.

Small as it may be, this trend benefits the fitness business. "People who come from other professions make some of the best trainers because they are doing it out of love," says Everett Aaberg, who educates trainers seeking certification through the Cooper Institute in Dallas. "A lot of the others are just old football players and muscleheads that don't know what else to do for a living."

A fitness trainer with a law degree can provide some high-minded conversation. Also, the advice from such a trainer isn't necessarily limited to weight lifting. "Sure, Jeff and I talked some dentistry," says Terry Mueller, a physical-fitness client of Mr. Knipper, the erstwhile dentist. "It was his idea that I get these porcelain laminates on my four front teeth."

Many trainers make their own hours and work for themselves, giving the gyms they use a cut -- as much as half -- of the proceeds. And they get to wear shorts and T-shirts all day.

Of course, there are trade-offs. A big one is respect. Mr. Chacon, the former attorney, felt like a caddie the first time a client stuffed extra cash into his hand. "I used to be the person who did the tipping," he says.

The Big-Lug Problem

Then there's the perception that fitness trainers boast more brawn than brains. "When people hear what you do, they wonder if you're intelligent, and I think: I've had more education than you," says Patti Heniff, a 35-year-old former schoolteacher with a graduate degree who switched to fitness training.

Not that there's no basis for that perception: It took Mr. Knipper eight years to earn his dental diploma. A fitness certification? "You can get that in a week if you cram for it," he says. To be sure, some certifying programs are rigorous and require many weeks of classes. Some trainers obtain undergraduate and graduate degrees in health and exercise sciences.

Even if trainers themselves don't care about public perceptions, their families sometimes do. When friends of Lorri Goergen inquire about her 27-year-old daughter, Mrs. Goergen replies, "Jennifer's a certified public accountant working toward a degree in kinesiology." Few know that kinesiology is the study of exercise. Fewer still know that Jennifer quit her accounting job at Grant Thornton LLP last year to work as a fitness trainer.

Says Jennifer: "Accounting wasn't for me."

Money Issues

The biggest trade-off is money. Trainers typically earn between $20,000 and $40,000 a year and often get no other benefits (though hard workers in high-fee gyms do better than that). This is why Brud Drachman, who much prefers his after-hours work as a fitness trainer to his duties as vice president of a San Diego company, won't quit his day job. "I make into six figures, and to replace even 80% of that as a personal trainer, I'd have to book every single hour of the week," he says.

Some who have made the switch hope to strike it rich off videos, television shows or by opening their own gyms. Others dream of good fortune. "I don't want to practice law again -- I just want to marry a guy who's wealthy," says Ms. Gresko, the Notre Dame-educated attorney.

She might make a good catch for any man who loves fit women. At last year's Hawaiian Ironman triathlon -- the world's most competitive endurance event -- she won the 35- to 39-year-old age group.

Not everyone counting clients' sit-ups considers this a true calling. Only after failing to earn fame and fortune as an actor did Jerry Pinkowski become a Chicago fitness trainer and gym owner. "Would I rather be a really successful actor?" asks Mr. Pinkowski, 45. "Yeah, I'd have to lean with that."