Four hundred thirty patients with Parkinson's disease were treated with amantadine hydrochloride alone or in conjunction with other anti-Parkinsonism medications over a 2 1/2-year period. Of 351 patients who took 200 mg a day for 60 days, 64% showed a favorable response to the drug at 60 days and thereafter. Of those patients who received amantadine and levodopa, 79% responded favorably, as compared with 48% in the group who did not receive levodopa. In one half of the patients treated, a decline in the therapeutic efficacy of amantadine was observed between 30 and 60 days. Few side effects were noted even after long periods of administration, and laboratory studies disclosed no consistent significant abnormalities.