We have studied disease progression in a group of 10 patients with recent onset idiopathic Parkinson's disease [mean age 53.7 +/- 12.6 years, mean duration 18.3 +/- 6.5 months, mean unified Parkinson's disease rating scale (UPDRS) 19.0 +/- 9.5], and a group of seven patients with established Parkinson's disease (mean age 60.1 +/- 15.1 years, mean duration 70.8 +/- 35.5 months, mean UPDRS 47.9 +/- 18.1), using both clinical assessment and [18F]dopa-PET. Results were compared with those of a group of 10 normal subjects (age 66 +/- 16 years). Mean putamen [18F]dopa influx constant (Ki) was a reliable measurement (R = 0.82) and the most sensitive marker of disease progression (r = -0.85, P < 0.0001). The mean annual rate of reduction in mean putamen Ki in the Parkinson's disease patients was 12.5% per annum, whereas the control group showed no significant change in Ki over a mean of 32 months follow up. The rate of progression was more rapid in the recent onset compared with the established disease group but this did not reach statistical significance. Assuming a linear progression for the entire group we estimate symptom onset at a mean putamen Ki 79% of normal with a mean preclinical period of 3.1 years.