Ciladopa is a partial dopamine agonist that is effective in patients with advanced Parkinson's disease who are no longer satisfactorily responding to levodopa. Thirty-one patients participated in a double-blind randomized study of ciladopa (added to levodopa) versus placebo. Among 21 patients randomized to treatment with ciladopa and levodopa, there was a 32% decrease in symptoms on the Modified Columbia University Disability Scale. This change was significant, p less than or equal to 0.05. Eight of the 21 patients (38%) improved by at least 50%. The mean number of hours "on" increased by 20%. This change was significant, p less than or equal to 0.05. Five of the 21 patients (24%) were on for at least 4 hours more than at baseline. Dyskinesias were not increased. The mean dose of ciladopa was 19.5 mg/d. The mean dose of levodopa in Sinemet was decreased by 10%. Studies with ciladopa in humans had to be discontinued because of the occurrence of microscopic testicular tumors in some rodents. Although improvement in patients taking ciladopa was modest, there were few adverse effects. These results are encouraging, because two other partial agonists are now available, and they may be as effective as ciladopa.