The effect of levodopa on Parkiruon's disease and syndrome was studied in a total of 1,533 patients according to a protocol for open clinical evaluation. The average daily dose to achieve maximum relief of clinical symptoms with tolerable side effects ranged from between 3.5 and 6.5 Gm. per day given in divided doses. The dosage of levodopa, sometimes administered concomitantly with other antiparkinson agents, was increased slowly in small increments, and dosage regimens were individualized for each patient according to his response. Approximately two thirds of the patients treated achieved a satisfactory resporue, while side effects occurred in over 90 per cent of the patients treated. Improvement in clinical symptomatology was sometimes delayed but continued on long‐term maintenance therapy. Laboratory abnormalities, though frequent, were rarely serious and, to date, are reversible. An additional 3,000 patients were monitored for severe and unusual reactioru and deaths. No death in any of the studies was attributed directly to the administration of the drug; however, experience showed that psychiatric changes masked by Parkinson's disease may manifest themselves in depression with suicidal tendencies as therapeutic doses are approached. It is concluded that, with proper care and careful management, levodopa appears to be a relatively safe and effective agent for the treatment of Parkinson's disease and syndrome.