OBJECTIVE: To evaluate whether type 2 diabetes at baseline is a risk factor for Parkinson's disease. RESEARCH DESIGN AND METHODS: We prospectively followed 51,552 Finnish men and women 25-74 years of age without a history of Parkinson's disease at baseline. History of diabetes and other study parameters were determined at baseline using standardized measurements. Ascertainment of the Parkinson's disease status was based on the nationwide Social Insurance Institution's drug register data. Hazard ratios of incident Parkinson's disease associated with the history of type 2 diabetes were estimated. RESULTS: During a mean follow-up period of 18.0 years, 324 men and 309 women developed incident Parkinson's disease. Age- and study year-adjusted hazard ratios of incident Parkinson's disease among subjects with type 2 diabetes, compared with those without it, were 1.80 (95% CI 1.03-3.15) in men, 1.93 (1.05-3.53) in women, and 1.85 (1.23-2.80) in men and women combined (adjusted also for sex). Further adjustment for BMI, systolic blood pressure, total cholesterol, education, leisure-time physical activity, smoking, alcohol drinking, and coffee and tea consumption affected the results only slightly. The multivariate adjusted association between type 2 diabetes and the risk of Parkinson's disease was also confirmed in stratified subgroup analysis. CONCLUSIONS: These data suggest that type 2 diabetes is associated with an increased risk of Parkinson's disease. Surveillance bias might account for higher rates in diabetes. The mechanism behind this association between diabetes and Parkinson's disease is not known.