To evaluate the frequency of glucocerebrosidase (GBA) mutations in cases and controls enrolled in the Genetic Epidemiology of Parkinson's Disease (GEPD) study.We sequenced all exons of the GBA gene in 278 Parkinson disease (PD) cases and 179 controls enrolled in GEPD, with a wide range of age at onset (AAO), and that included a subset of 178 Jewish cases and 85 Jewish controls. Cases and controls were recruited without knowledge of family history of PD, and cases were oversampled in the AAO 50 years (OR 2.7, 95% CI 1.3 to 5.3). Adjusting for age at the time of evaluation, sex, family history of PD, and Jewish ancestry, GBA carriers had a 1.7-year-earlier AAO of PD (95% CI 0.5 to 3.3, p 50 years group. The frequency of GBA mutations was higher in a subset of 178 cases that reported four Jewish grandparents (16.9%) than in cases who did not report Jewish ancestry (8.0%) (p < 0.01). Nine different GBA mutations were identified in PD cases, including 84insGG, E326K, T369M, N370S, D409H, R496H, L444P, RecNciI, and a novel mutation, P175P.This study suggests that the Glucocerebrosidase gene may be a susceptibility gene for Parkinson disease and that Glucocerebrosidase mutations may modify age at onset.