Dopamine is involved in the regulation of food intake, and obese persons have decreased dopamine D2 receptor availability in the striatum. Furthermore, midlife triceps skinfold thickness has been found to be positively associated with the risk of Parkinson's disease (PD) among Japanese-American men in Hawaii. The authors prospectively investigated whether obesity was associated with PD risk in two large cohorts of US men and women. They documented 249 cases of PD in men (1986-2000) and 202 cases in women (1976-1998). Neither baseline body mass index (weight (kg)/height (m)(2)) nor early adult body mass index was associated with PD risk. The multivariate relative risk for a baseline body mass index of > or = 30 versus <23 was 0.8 (95% confidence interval (CI): 0.6, 1.2; p for trend = 0.3). Overall, waist circumference and waist-to-hip ratio were not related to PD risk. However, among never smokers, both variables showed significantly positive associations with PD risk. The relative risks for comparisons of extreme quintiles were 1.9 (95% CI: 1.0, 3.4; p for trend = 0.03) for waist circumference and 2.0 (95% CI: 1.1, 3.6; p for trend = 0.03) for waist-to-hip ratio. The results do not support a role of overall obesity in PD pathogenesis; however, central obesity may be associated with higher PD risk among never smokers, and this finding merits further investigation.