An analysis of 84,170 participants in the California Men’s Health Study was performed to investigate the association between Cannabis use and the incidence of bladder cancer. During 16 years of follow-up, 89 Cannabis users (0.3%) developed bladder cancer compared with 190 (0.4%) of the men who did not report Cannabis use (P < .001). After adjusting for age, race, ethnicity, and body mass index, Cannabis use was associated with a 45% reduction in bladder cancer incidence (hazard ratio, 0.55; 95% confidence interval, 0.33–1.00).