On a snowy January evening 12 years ago, six women gathered in a South Orange, N.J., church basement to take an exercise-to-music class. The instructor: an energetic former professional dancer named Jacki Sorensen.

From that humble beginning, Sorensen--part priestess, part therapist and a dash of drill sergeant--has endeared herself to thousands of hearts and thighs around the world. Her Aerobic Dancing, Inc., has exploded into a multimillion dollar business with 4,000 instructors and 150,000 students in 44 states and several foreign countries.

Sorensen is choreographer, author and chairman of the board of the California-based firm, which has spawned countless spin-offs and inspired the entire new field of "dancersize": a catch-all term for movement-to-music shape-ups ranging from Jane Fonda's "Workout" to Disneyland's "Mousercise."

Everyone, it seems, is gyrating to one beat or another.

Aerobic dancing's success "might be a sign of the times," says Sorensen, 40, who danced through town recently to promote her latest fitness tome and teach "life-style classes" to legions of devoted fans.

"In the last Depression people went to the movies to see the musical comedies. Now we're putting on a musical comedy for them to dance in and entertain themselves."

This "fun factor" is central to aerobic dancing's popularity, says Sorensen. Her program stems from a TV exercise show she was asked--as a dancing teacher--to create in 1969 by Air Force base producers in Puerto Rico, where her husband was stationed as a pilot. (They subsequently moved to New Jersey.)

"At first I thought 'no way,' because dance exercises are the most boring part of dancing," recalls Sorensen, "but I said I'd look into it."

Her research led her to a new, relatively obscure book--the Air Force aerobics program by Dr. Kenneth Cooper--now considered the bible of the fitness movement.

"No one knew about aerobics back then," she says, "but Ken Cooper made so much sense. I took his 12-minute running test and scored 'excellent' even though I'd never run before.

"When I realized that years of dance training had kept my heart and lungs in shape, I knew I was onto something."

That "something" was the now-famous combination of cardiovascular conditioning with the flexibility, toning and fun of dance routines choreographed to popular music.

In addition to the overt physical benefits, aerobic dancing also offers a mental-health boost "from the subtle spirit I transmit through the dances." Her message: "Optimism and love and patriotism and success."

"Yes," she says, "we're shaping up the inner and outer thighs, and yes we take care of heart rates. But within it all I'm saying 'Have faith in yourself,' 'You can do it' .

"When you get 50 women together, dancing with a high heart rate singing 'There's no stopping us now,' it's hard not to cry.

"The response on this tour has been incredible. There were 1,300 in the Boston class and over a 1,000 in Towson Md. ."

How does she react to the clamor among fans? "I don't even think about that aspect of it. I have a message, that's all. Some people paint, I choreograph."

Sometimes, she admits, "I have to force that optimistic feeling. I went through a personal tragedy last year when my 2-year-old niece drowned. The family went berserk. I wanted to dedicate a session to little Tamara so I picked music that I wanted to believe in."

She titled the 12-week, 15-routine session "Harmony" and chose music such as "I believe in music, I believe in love"; "Love's been a little hard on me"; "I got the music in me," and "Oh lift me up where I belong."

"When I opened myself up to my students that way," she says, "they started to write me about how aerobic dancing helped them through their hard times. They're going through divorces or they're at the age where their parents are dying. It's been very touching.

"I really didn't believe in that session when I choreographed it, but I knew I needed it. And now I feel, 'Yes, I do believe that.' "

The current session is called "Celebrate Life." "I've been through the healing process," she says. "My sister is pregnant again."

Sorensen's dream now is to host "a really uplifting TV show. I don't want to get burned out. But I'll never get tired of choreography. It's my own private experience to get up, look at the mirror and say 'Okay, how are we going to entertain those troops today?' "

Sorensen's newest project is Aerobic Workout, a "less chorus line, more athletic" program designed to attract more men to her classes. For the first time, Neil, her husband of 18 years and president of Aerobic Dancing, Inc., is taking classes. "He wears this T-shirt that says 'Jane Fonda Does Not Work Out Here.' It's a scream."

When asked about Fonda and others now touting their own exercise-to-music programs Sorensen is pointedly polite.

"It's flattering. Aerobic dancing isn't for everyone. There's room on the market for a variety of options. There are still 40 million people doing nothing."

While she won't come out and call other programs harmful, she does say, "I am a little bit concerned about unsafe exercises in some programs. I want to educate people to ask teachers about their training. After all, you're putting your body in their hands."

Also, she says, "There's a lot of confusion among the public because some courses called 'aerobic' just aren't. You can't jog in place three minutes and claim real aerobic benefit. It's got to be 20 minutes.

"What I'm trying to do with my book is get people to upgrade their programs and strive to be the high-quality program that we are."

So far as Fonda and "the burn," Sorensen says, "I don't believe you need to take a muscle to an exhausted quiver point to make it stronger. Fitness has to be fun or you're adding stress to your life. And who needs that?"

If some disparage Sorensen's outlook as saccharine and unrealistically rosy, she frankly doesn't care. "I don't mind getting teased about being Suzy Sunshine because I'm secure in knowing I'm a complete person as much as I can be. I feel there's a need not to always focus on the negative.

"It's like that Neil Diamond song where he says, 'Look around and there's this bad thing and that bad thing'; then the chorus goes, 'But every day there's a brand new baby born, there's a sun to keep you warm.'

"I don't think that's Pollyanna. It shows both sides. There's a choice. I chose the positive."