Parkinson's disease, which robs people of control over their movements, usually starts in middle or late life and develops slowly.

Some 500,000 to 1 million Americans are thought to have the condition, which can include such symptoms as tremor, slow movement, shuffling gait, stiff limbs and difficulty in keeping balance.Symptoms may appear on one or both sides of the body.

Attorney General Janet Reno announced Thursday she has the disease and is taking Sinemet, which contains levodopa, the standard medication for Parkinson's. The brain turns levodopa into dopamine, and since Parkinson's is caused by a shortage of dopamine in a key brain region, the medication eases symptoms.

While levodopa treatment is effective, it can lose its effect over time. So researchers have been studying alternative treatments, such as implanting brain cells to pump out dopamine.

Reno, 57, said she feels fit and has no intention of leaving office. "As I grow old and become a very old lady, I may find limitation in mobility, limitation in muscle response, but I feel fine now," she said.

Reno said she had not discussed her ailment with President Clinton because she did not want to bother him at this time. But she said the White House chief of staff, Leon Panetta, had been "very supportive."

Dr. Raymon Durso, director of the Parkinson's Disease Clinic at the Boston Veterans Affairs Medical Center, said Reno's comments suggested that she has very early and mild Parkinson's.

"Patients typically can go years with mild Parkinson's disease, functioning at their previous level, on medication," he said.

Parkinson's itself does not kill, but eventually when medication loses its effect a patient can become bedridden and could die from the complications, Durso said.