Importance Previous studies have linked suicide risk with religious participation, but the majority have used ecologic, cross-sectional, or case-control data. Objective To examine the longitudinal association between religious service attendance and suicide and the joint associations of suicide with service attendance and religious affiliation. Design, Setting, and Participants We evaluated associations between religious service attendance and suicide from 1996 through June 2010 in a large, long-term prospective cohort, the NursesÕ Health Study, in an analysis that included 89 708 women. Religious service attendance was self-reported in 1992 and 1996. Data analysis was conducted from 1996 through 2010. Main Outcomes and Measures Cox proportional hazards regression models were used to examine the association between religious service attendance and suicide, adjusting for demographic covariates, lifestyle factors, medical history, depressive symptoms, and social integration measures. We performed sensitivity analyses to examine the influence of unmeasured confounding. Results Among 89 708 women aged 30 to 55 years who participated in the NursesÕ Health Study, attendance at religious services once per week or more was associated with an approximately 5-fold lower rate of suicide compared with never attending religious services (hazard ratio, 0.16; 95% CI, 0.06-0.46). Service attendance once or more per week vs less frequent attendance was associated with a hazard ratio of 0.05 (95% CI, 0.006-0.48) for Catholics but only 0.34 (95% CI, 0.10-1.10) for Protestants (P = .05 for heterogeneity). Results were robust in sensitivity analysis and to exclusions of persons who were previously depressed or had a history of cancer or cardiovascular disease. There was evidence that social integration, depressive symptoms, and alcohol consumption partially mediated the association among those occasionally attending services, but not for those attending frequently. Conclusions and Relevance In this cohort of US women, frequent religious service attendance was associated with a significantly lower rate of suicide.