Low levels of dopamine, a brain chemical (neurotransmitter) involved in controlling movement, cause symptoms of Parkinson's disease. The shortage of this brain chemical occurs when nerve cells in a part of the brain (substantia nigra) that produces dopamine fail and deteriorate. The exact cause of this deterioration is not known.

The links between Parkinson's disease and factors such as genetics, aging, toxins in the environment, and free radicals are all under investigation. Although these studies are beginning to provide some answers, experts do not know the exact cause of the disease.

Studies are ongoing to determine whether there is a genetic cause of Parkinson's disease. Only a small percentage of people with Parkinson's disease have a parent, brother, or sister who has the disease; however, abnormal genes do seem to be a factor in a few families where early-onset Parkinson's disease is common.