Thirty-four patients have been studied from the time of initiation of pharmacological treatment in a long-term prospective study of levodopa effects and disease progression in Parkinson s disease. Objective motor scoring of the response to levodopa in defined off states was performed every 3 years. The mean time from the initiation of levodopa treatment to the most recent measurements was 18.2 years. Of 8 patients who are still alive, only 3 had none of the features of the advanced disease phase (dementia, hallucinations, frequent falling). Off-phase motor function worsened at a yearly rate of 1.9 of the maximum disability score, although the plots of the serial scores showed that the magnitude of the levodopa response is well preserved. There was little difference in the rate of progression between patients with tremor-dominant and non-tremor-dominant motor subtypes. Those who developed dementia had more rapid deterioration of motor scores, with significantly worse off-phase (P = .008) and on-phase (P = .03) motor function. A graph of serial scores of patients who have died, aligned for time of death, showed an upward curving trend of motor disability in the last 5 years of the disease course. Its advanced phase may reveal that Parkinson s disease has an exponential pattern of progression.