The authors prospectively examined bowel movement frequency at baseline in relation to future Parkinson's disease risk in the Health Professionals Follow-up Study (HPFS) during 2000-2006 (33,901 men) and the Nurses' Health Study (NHS) during 1982-2006 (93,767 women). During the follow-up (6 years for the HPFS and 24 years for the NHS), the authors identified 156 incident male Parkinson's disease cases (HPFS) and 402 female cases (NHS). In the HPFS, compared with men with daily bowel movements, men with a bowel movement every 3 days or less had a multivariate-adjusted relative risk of 4.98 (95% confidence interval (CI): 2.59, 9.57) for developing Parkinson's disease in the next 6 years. In the NHS, the corresponding relative risk was 2.15 (95% CI: 0.76, 6.10), and the risk of Parkinson's disease was not elevated beyond 6 years of follow-up (relative risks = 1.25 for years 7-12, 0.54 for years 13-18, and 0.88 for years 19-24). When these 2 cohorts were combined, the pooled relative risks for Parkinson's disease in the next 6 years were 0.75, 1 (referent), 2.62, and 3.93 (95% CI: 2.26, 6.84) (P(trend) < 0.0001) across 4 bowel movement categories. In conclusion, infrequent bowel movements may antedate the onset of cardinal motor symptoms of Parkinson's disease and may contribute to the identification of populations with higher than average Parkinson's disease risk.