Erectile dysfunction is common among individuals with Parkinson's disease, but it is unknown whether it precedes the onset of the classic features of Parkinson's disease. To address this question, the authors examined whether erectile dysfunction was associated with Parkinson's disease risk in the Health Professionals Follow-up Study. Analyses included 32,616 men free of Parkinson's disease at baseline in 1986 who in 2000 completed a retrospective questionnaire with questions on erectile dysfunction in different time periods. Relative risks were computed using Cox proportional hazards models adjusting for age, smoking, caffeine intake, history of diabetes, and other covariates. Among men who reported their erectile function before 1986, 200 were diagnosed with Parkinson's disease during 1986-2002. Men with erectile dysfunction before 1986 were 3.8 times more likely to develop Parkinson's disease during the follow-up than were those with very good erectile function (relative risk = 3.8, 95% confidence interval: 2.4, 6.0; p < 0.0001). Multivariate-adjusted relative risks of Parkinson's disease were 2.7, 3.7, and 4.0 (95% confidence interval: 1.4, 11.1; p = 0.008) for participants with first onset of erectile dysfunction (before 1986) at 60 or more, 50-59, and less than 50 years of age, respectively, relative to those without erectile dysfunction. In conclusion, in this retrospective analysis in a large cohort of men, the authors observed that erectile dysfunction was associated with a higher risk of developing Parkinson's disease.