BACKGROUND: We have demonstrated that obstructive sleep apnea (OSA) is prevalent in 60% of patients undergoing bariatric surgery. A study was conducted to determine whether weight loss following bariatric surgery ameliorates OSA. METHODS: All 100 consecutive patients with symptoms of OSA were prospectively evaluated by polysomnography before gastric bypass. Preoperative and postoperative scores of Epworth Sleepiness Scale (ESS), Respiratory Disturbance Index (RDI), and other parameters of sleep quality were compared using t-test. RESULTS: Preoperative RDI was 40 +/- 4 (normal 5 events/hour, n = 100). 13 patients had no OSA, 29 had mild OSA, while the remaining 58 patients were treated preoperatively for moderate-severe OSA. At a median of 6 months follow-up, BMI and ESS scores improved (38 +/- 1 vs 54 +/- 1 kg/m2, 6 +/- 1 vs 12 +/- 0.1, P < 0.001, postoperatively vs preoperatively). To date, 11 patients have completed postoperative polysomnography (3-21 months) after losing weight (BMI 40 +/- 2 vs 62 +/- 3 kg/m2, P < 0.001). There was significant improvement in ESS (3 +/- 1 vs 14 +/- 2), minimum O2 saturation (SpO2 86 +/- 2 vs 77 +/- 5), sleep efficiency (85 +/- 2% vs 65 +/- 5%), all P < 0.001, postop vs preop; and RDI (56 +/- 13 vs 23 +/- 7, P = 0.041). Regression analysis demonstrated no correlation between preoperative BMI, ESS score and the severity of OSA; and no correlation between % excess body weight loss and postoperative RDI. CONCLUSION: Weight loss following gastric bypass results in profound improvement in OSA. The severity of apnea cannot be reliably predicted by preoperative BMI and ESS; therefore, patients with symptoms of OSA should undergo polysomnography.