High body mass index has been associated with increased risk of several chronic diseases, including cardiovascular disease, and, recently, Alzheimer's disease. There are few data on the association of body mass index with Parkinson's disease, and results have been inconsistent. The authors conducted a prospective study among 10,812 men in the Harvard Alumni Health Study, followed from 1988 to 1998 (mean age at baseline: 67.7 years), to test the hypothesis that body mass index is associated with Parkinson's disease risk. Among 106 incident cases of Parkinson's disease, body mass index at baseline was not associated with Parkinson's disease risk (for body mass index <22.5, 22.5-<24.9, and > or =25.0 kg/m2: multivariate relative risks = 1.51 (95% confidence interval: 0.95, 2.40), 1.00 (referent), and 0.86 (95% confidence interval: 0.53, 1.41)). The authors had information on body mass index during late adolescence, when men entered college; this was unrelated to Parkinson's disease risk as well. Subjects who lost at least 0.5 units of body mass index per decade between college entry and 1988 had a significantly increased Parkinson's disease risk, compared with men having stable body mass index (multivariate relative risk = 2.60, 95% confidence interval: 1.10, 6.10). The authors conclude that body mass index is unrelated to Parkinson's disease risk and speculate that the observation of increased risk with body mass index loss since late adolescence may reflect weight loss due to Parkinson's disease that preceded clinical diagnosis.