Breaking seven years of silence, Michael J. Fox has revealed he has Parkinson's disease.

"It's not that I had a deep, dark secret," the 37-year-old actor, best known for his roles in the "Back to the Future" movies and the old sitcom "Family Ties," says in the Dec. 7 issue of People magazine. "It was just my thing to deal with."

By talking about the incurable disease, Fox feels he can help himself, his family -- he's married to actress Tracy Pollan, 38, and they have a 9-year-old son and 3-year-old twin daughters -- and other sufferers of the disease.

Parkinson's disease, a degenerative neurological disorder, destroys brain cells that produce dopamine, a chemical important for movement. Symptoms include muscle stiffening, tremors and difficulty moving.

The disease affects about 1 million Americans, most older than 60. Former heavyweight boxing champ Muhammad Ali and Attorney General Janet Reno both suffer from it.

Fox, who currently stars on the ABC sitcom "Spin City," says the trembling in his left arm and leg was so bad last January that he asked his limo driver to circle the block three times before dropping him off at the Golden Globe Awards.

In March, Fox underwent a thalamotomy, a form of brain surgery.

Fox's spokeswoman, Nanci Ryder, said Fox will continue to work on "Spin City," and an interview with Barbara Walters will air on Dec. 4.

In 1991, Fox noticed a "twitch" in his left pinkie while on the set of the film "Doc Hollywood." After undergoing tests, he received the diagnosis.

"It was incomprehensible," he says in the issue of People that hits newsstands Friday. "The doctor said I would be able to function for years and years." But over the next few years, the disease progressed, and his entire left side suffered from stiffness and tremors. "And I mean big tremors," says Fox, who can still joke about himself. He says he suffered a shaking of his left arm so violent, "I could mix a margarita in five seconds."

Actress Meredith Baxter, who played Fox's television mother on "Family Ties," applauded his decision to go public.

"The fact that Michael is passing along his experience and truth is a very courageous and loving thing to do," she said in statement released today. CAPTION: Fox, 37, star of TV's "Spin City," said he has known about his illness for seven years. ec