Transient exposure to the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a syndrome resembling idiopathic parkinsonism (IP). While IP inevitably progresses, the long-term evolution of MPTP-parkinsonism is unknown. Fluorodopa positron emission tomography (FD-PET) is a reliable tool for assessing nigrostriatal dopaminergic function. We performed FD-PET and clinical assessments on two occasions, 7 years apart, on 10 human subjects exposed to MPTP (age at the first scan, 32.7 +/- 6.9 yr [mean +/- SD], and on 10 normal individuals (age, 53 +/- 16 yr). At the time of their first scan, 5 of the subjects exposed to MPTP were clinically normal and 5 had limited signs of parkinsonism; 5 had new clinical deficits 7 years later. In the subjects exposed to MPTP, the PET index [(striatal-occipital)/occipital ratio] dropped by 2.3% per year from 0.70 +/- 0.10 (mean +/- SD) to 0.58 +/- 0.10 (p < 0.001). This was significantly faster than normal aging (p < 0.01) and similar to the progression observed in IP (p = 0.06). The findings suggest that short-term exposure to MPTP leads to a protracted decline in nigrostriatal dopaminergic function more rapid than occurs in normal aging and similar to IP progression. This is the first evidence that transient exposure to a toxin can cause progressive nigral pathology. At present, the mechanism leading to this progression is unknown. Our findings support the hypothesis that some neurodegenerative disorders may result from transient exposure to an environmental agent.