Among 406 patients with Parkinson's disease, the cancer rate (all sites combined) was bout one-third that for the general population. The risk of cancer increased during the treatment period but remained significantly low. Malignant and benign thyroid neoplasms were significantly more frequent than expected among patients with Parkinson's disease. We suggest that high levels of total body potassium in patients with Parkinson's disease is the protective factor against cancer.