Visitors to Gleason’s Gym won’t find flat-screen televisions hanging from the rafters or stacks of fluffy towels lining the countertops. There’s no smoothie bar or day spa in sight at this venerable boxing gym, where the barbells are rusting and the weight-lifting benches are held together with masking tape.

The 71-year-old institution, founded in the Bronx and now situated in Brooklyn’s DUMBO neighborhood, prides itself not on technology or amenities, but on the scores of champion prizefighters who have trained there. Among them are Jake “the Bronx Bull” LaMotta, Carlos Ortiz, Cassius Clay (who later changed his name to Muhammad Ali), and Mike Tyson. Actress Hilary Swank trained at Gleason’s in preparation for her Oscar-winning role in the 2004 film “Million Dollar Baby.”

Even as it turns out top-ranked boxers and trains onscreen prizefighters, Gleason’s — billed as the nation’s oldest active boxing gym — is seeing an influx of fitness-focused clients who have taken boxing-inspired aerobics classes at upscale health clubs. “They get an introduction to the sport, and then they say, ‘What’s next?'” the owner, Bruce Silverglade, said. “Increasingly, health clubs are training grounds for fight gyms like ours.”

Making that transition from more genteel chains or boutique gyms means forgoing all frills; it means training in a space where the grunting is loud and the odor of sweat is pungent. Mr. Silverglade, who left a corporate job at Sears 25 years ago to become the third person ever to own the gym, boasts of Gleason’s rough-and-tumble atmosphere. He points out the peeling paint, the threadbare furniture, and the decades-old machines during a recent tour of the 18,000-square-foot facility, which is a popular, retro backdrop for feature films and fashion shoots. “I’m a traditionalist,” he said. “You don’t need a fancy gym to get conditioned and to train for a boxing competition.”

For nearly a decade, 33-year-old Soozan Baxter belonged to a national chain gym, where she would occasionally spar with her personal trainer and take group kickboxing classes. She joined Gleason’s about two years ago, upon moving to Manhattan from Baltimore.

“This is a much more serious place to train, but my friends would probably die of shock if they saw the place,” Ms. Baxter, the vice president of luxury brand leasing for a shopping mall developer, said. “It’s dirty, it smells, and when you’re fighting, the other person’s sweat is all over you.”

Contending with the grime is a small trade-off for her thrice-weekly Gleason’s workouts, Ms. Baxter said, though she still won’t venture into the gym’s locker rooms. “It’s the biggest stress reliever in the entire world,” she said. “If you’re having a bad day at work or just woke up on the wrong side of the bed, you can channel that aggression in a way that you can’t with Pilates or push-ups.”

She calls Gleason’s the “last great bargain in New York City.” With private boxing lessons running $35 an hour, it’s about a third to a quarter the price of a personal training session at many fitness centers.

A commercial real estate manager from South Hempstead, L.I., Maureen Reininger, decided to take up boxing about three years ago — after having attempted hot yoga, Pilates, and a range of aerobics classes at area health clubs.

She said boxing has proved far more physically demanding than any other sport or workout she had tried previously. “I’ve become a lot more toned,” Ms. Reininger, who plans to compete on the amateur circuit in a division for boxers ages 35 and up, said. “I have a lot more endurance, a lot more stamina, and a lot more confidence. Just the way I walk, the way I move, is different.”

On a recent afternoon, Ms. Reininger was being coached for an upcoming fight by the 29-year-old world welterweight champion, Belinda Laracuente, who moved to New York from Florida to train at Gleason’s for her title.

Training nearby was 15-year-old Mateusz Szczepanski and his 13-year-old sister, Ada, who come to Gleason’s two afternoons a week. They spend two hours conditioning — jumping rope, running on the treadmill, lifting weights, punching heavy bags — before taking an hour-long private lesson with their mother, Agnieszka Szczepanski, sitting ringside. While their peers may be watching television or playing video games, “they’re moving for three hours straight,” Ms. Szczepanski, who lives in Brooklyn, said.

Opened in 1937 by Robert Gagliardi — he called the gym Gleason’s, not Gagliardi’s, to appeal to a fight crowd that was, at the time, predominantly Irish — the gym now thrives on the diversity of its 1,500-person clientele. On any given day, a mélange of amateur and professional boxers, professional wrestlers, students of all ages, teachers, lawyers, and financiers are likely to populate Gleason’s four rings.

“We have Israelis training next to Palestinians; we have police officers training next to the people they arrested last week; we have Ph.D.s training next to high school dropouts,” Mr. Silverglade said. “Here, you can’t tell who’s who.”

Gleason’s Gym, 77 Front St. at Main Street, Brooklyn, 718-797-2872, $85 a month; private boxing lessons, $35 an hour. Gleason’s also sponsors Fantasy Camp weekend retreats. For more information, visit gleasonsgym.net.