The authors prospectively examined whether caffeine intake was associated with lower risk of Parkinson disease (PD) in both men and women among 304,980 participants in the National Institutes of Health-AARP Diet and Health Study and whether smoking affected this relation. Multivariate odds ratios and 95% confidence intervals were derived from logistic regression models. Higher caffeine intake as assessed in 1995-1996 was monotonically associated with lower PD risk (diagnosed in 2000-2006) in both men and women. After adjustment for age, race, and physical activity, the odds ratio comparing the highest quintile of caffeine intake with the lowest was 0.75 (95% confidence interval: 0.60, 0.94; P(trend) = 0.005) for men and 0.60 (95% confidence interval: 0.39, 0.91; P(trend) = 0.005) for women. Further adjustment for duration of smoking and analyses carried out among never smokers showed similar results. A joint analysis with smoking suggested that smoking and caffeine may act independently in relation to PD risk. Finally, the authors conducted a meta-analysis of prospective studies and confirmed that caffeine intake was inversely associated with PD risk in both men and women. These findings suggest no gender difference in the relation between caffeine and PD.