An analysis of patients undergoing laparoscopic Roux-en-Y gastric bypass (RYGB) may identify factors predictive of complication and of suboptimal weight loss. Inception cohort. Metropolitan university hospital. One hundred eighty-eight consecutive patients with severe obesity who met National Institutes of Health consensus guidelines for bariatric surgery. Laparoscopic RYGB. Complications requiring therapeutic intervention and percentage of excess body weight lost at 1 year after surgery. Of the 188 patients who underwent laparoscopic RYGB, 50 (26.6%) developed complications that required an invasive therapeutic intervention, including 2 deaths. The average follow-up was 351 days (range, 89-1019 days). Multivariate analysis by stepwise logistic regression identified surgeon experience, sleep apnea (P =.003; odds ratio, 3.0; 95% confidence interval, 1.3-7.1), and hypertension (P =.07; odds ratio, 2.0; 95% confidence interval, 1.0-4.0) as predictors of complications. The most common complication requiring therapeutic intervention was stricture at the gastrojejunal anastomosis, occurring in 27 patients (14.4%). Of the 115 patients who underwent surgery more than 1 year previously, 1-year follow-up data were available for 93 (81%). The body mass index (weight in kilograms divided by the square of height in meters) decreased from 53 +/- 8 preoperatively to 35 +/- 6 at 1 year. The mean +/- SD percentage of excess body weight lost at 1 year was 61% +/- 14%. Diabetes mellitus was negatively correlated with percentage of excess body weight lost at 1 year (P =.06). Surgeon experience, sleep apnea, and hypertension are associated with complications after laparoscopic RYGB. Diabetes mellitus may be associated with poorer postoperative weight loss.