Background: Gastric bypass has profound effects on glycemic control in adults with type 2 diabetes mellitus. This study examined the long-term rates and clinical predictors of diabetes remission and relapse among patients undergoing gastric bypass. Methods: A retrospective cohort study of adults with uncontrolled or medication-controlled type 2 diabetes who underwent gastric bypass from 1995 to 2008 in three integrated health care delivery systems in the USA. Remission and relapse events were defined by diabetes medication use and clinical laboratory measures of glycemic control. We identified 4,434 adults with uncontrolled or medication-controlled type 2 diabetes who had gastric bypass. Results: Overall, 68.2 % (95 % confidence interval [CI], 66 and 70 %) experienced an initial complete diabetes remission within 5 years after surgery. Among these, 35.1 % (95 % CI, 32 and 38 %) redeveloped diabetes within 5 years. The median duration of remission was 8.3 years. Significant predictors of complete remission and relapse were poor preoperative glycemic control, insulin use, and longer diabetes duration. Weight trajectories after surgery were significantly different for never remitters, relapsers, and durable remitters (p = 0.03). Conclusions: Gastric bypass surgery is associated with durable remission of type 2 diabetes in many but not all severely obese diabetic adults, and about one third experience a relapse within 5 years of initial remission. More research is needed to understand the mechanisms of diabetes relapse, the optimal timing of surgery in effecting a durable remission, and the relationship between remission duration and incident microvascular and macrovascular events.