In a prospective population-based cohort study among 4,695 participants aged 55 years and older, with repeated in-person examination and on average 9.4 years of follow-up, we observed that higher serum levels of uric acid were associated with a significantly decreased risk of Parkinson disease (adjusted hazard ratio per standard deviation increase 0.71 [95% confidence interval 0.51-0.98]), with evidence for a dose-effect relationship (p value for trend over quartiles 0.040). Our findings support the hypothesis that oxidative stress contributes to the risk of Parkinson disease and suggest a potential protective effect of the natural antioxidant and free radical scavenger uric acid.