Our experience in treating 7 patients with moderate to severe parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is reviewed. Virtually all of the problems typically encountered with dopamine precursor and agonist therapy in treating Parkinson's disease have been observed during a one and one half year follow-up period, including "end-of-dose" deterioration (or "wearing off"), "peak-dose" dyskinesias, "on-off" phenomena, and psychiatric complications. These have occurred much earlier than is typically seen when treating the idiopathic disease. This rapid evolution of therapeutic side-effects favors the view that at least some of the complications of dopamine precursor therapy may be related to severity of disease rather than the length of levodopa therapy. Finally, we suggest that the occurrence of this full array of therapeutic complications in patients with MPTP-induced parkinsonism furthers the analogy between this syndrome ane Parkinson's disease.