Between 1991 and 2000, the prevalence of obesity increased 65 per cent. As a result, increasing research is being directed at gastric bypass (GB) surgery, an operation that appears to achieve long-term weight reduction. Despite the rapid proliferation of this surgery, the quality of care at a population level is largely unknown. This study examines longitudinal trends in quality and identifies significant predictors of adverse outcomes. Using the California inpatient discharge database, all GB operations from 1996 to 2000 were identified. Demographic, comorbidity, complication, and volume data were obtained. Complications were defined as life-threatening cardiac, respiratory, or medical (renal failure or shock) events. Comorbidity was graded on a modified Charlson score. Annual hospital volume was categorized into four groups: < 50, 50-99, 100-199, and 200+ cases. Based on these data, we calculated longitudinal trends in complication rate and performed logistic regression to identify predictors of complications. A total of 16,232 patients were included. The average age was 41 years; 84 per cent were female, and 83.5 per cent were white. The complication rate was 10.4 per cent. Between 1996 and 2000, rates of cardiac and respiratory complications decreased while rates of medical complications remained unchanged. Complications were more likely in men [odd ratio (OR) = 1.69 compared to women] and in patients with comorbidities (OR = 1.60 for each additional comorbid disease). Furthermore, when examining the effect of volume, patients at very low (< 50) and low (50-99) volume hospitals were much more likely to have complications (OR = 2.72 and 2.70, respectively) compared to patients at high-volume hospitals (200+), even after controlling for differences in case-mix. The quality of care for obesity surgery has improved between 1996 and 2000. Despite operating on patients with more comorbidity, rates of cardiac and respiratory complications have decreased. Furthermore, this study identifies three independent predictors of complications: gender, comorbidity, and hospital volume. These findings are important initial steps toward improving quality in obesity surgery.