Deprenyl (selegiline) delays the need for levodopa therapy in patients with early Parkinson's disease, but the long-term benefits of this treatment remain unclear. During 1987 to 1988, 800 patients with early Parkinson's disease were randomized in the Deprenyl and Tocopherol Antioxidative Therapy of Parkinsonism trial to receive deprenyl, tocopherol, combined treatments, or a placebo and were then placed on active deprenyl (10mg/day). A second, independent randomization was carried out in early 1993 for 368 subjects who by that time had required levodopa and who had consented to continuing the deprenyl treatment (D subjects) or changing to a matching placebo (P subjects) under double-blind conditions. The first development of wearing off, dyskinesias, or on-off motor fluctuations was the prespecified primary outcome measure. During the average 2-year follow-up, there were no differences between the treatment groups with respect to the primary outcome measure (hazard ratio, 0.87; 95% confidence interval, 0.63, 1.19; p = 0.38), withdrawal from the study, death, or adverse events. Although 34% of D subjects developed dyskinesias and only 19% of P subjects did (p = 0.006), only 16% of D subjects developed freezing of gait but 29% of P subjects did (p = 0.0003). Decline in motor performance was less in D subjects than P subjects. Levodopa-treated Parkinson's disease patients who had been treated with deprenyl for up to 7 years, compared with patients who were changed to a placebo after about 5 years, experienced slower motor decline and were more likely to develop dyskinesias but less likely to develop freezing of gait.