Bariatric surgery is the most effective therapy for severe obesity. People with bipolar disorder have increased risk of obesity, yet are sometimes considered ineligible for bariatric surgery due to their bipolar disorder diagnosis. This study aimed to determine if bariatric surgery alters psychiatric course among stable patients with bipolar disorder. A matched cohort study (2006-2009) with mean follow-up of 2.17 years was conducted within Kaiser Permanente Northern California, a group practice integrated health services delivery organization that provides medical and psychiatric care to 3.3 million people. Participants were 144 severely obese patients with bipolar disorder who underwent bariatric surgery, and 1,440 control patients with bipolar disorder, matched for gender, medical center, and contemporaneous health plan membership. Controls met referral criteria for bariatric surgery. Hazard ratio for psychiatric hospitalization, and change in rate of outpatient psychiatric utilization from baseline to Years 1 and 2, were compared between groups. A total of 13 bariatric surgery patients (9.0%) and 153 unexposed to surgery (10.6%) had psychiatric hospitalization during follow-up. In multivariate Cox models adjusting for potential confounding factors, the hazard ratio of psychiatric hospitalization associated with bariatric surgery was 1.03 [95% confidence interval (CI): 0.83-1.23]. In fully saturated multivariate general linear models, change in outpatient psychiatric utilization was not significantly different for surgery patients versus controls, from baseline to Year 1 (-0.4 visits/year, 95% CI: -0.5 to 0.4) or baseline to Year 2 (0.4 visits/year, 95% CI: -0.1 to 1.0). Bariatric surgery did not affect psychiatric course among stable patients with bipolar disorder. The results of this study suggest that patients with bipolar disorder who have been evaluated as stable can be considered for bariatric surgery.