To determine the prevalence of obstructive sleep apnea (OSA) in extremely overweight adolescents and to examine the effect of significant weight loss on OSA severity. We reviewed the anthropometric and polysomnographic data on all extremely overweight adolescents who underwent laparoscopic Roux en Y gastric bypass surgery at Cincinnati Children's Hospital Medical Center from July 2001 to September 2004. Repeat polysomnograms were performed after significant weight loss. Comparisons were made between pre- and postoperative polysomnographic data. Nineteen of 34 patients (55%) who underwent bariatric surgery were diagnosed with OSA. Subsequent to surgery, 10 of these patients returned for follow-up polysomnographic testing. After significant weight loss (mean, 58 kg), OSA severity markedly decreased in all patients (median apnea-hypopnea index at baseline vs. after weight loss, 9.1 vs. 0.65). Our study indicated that OSA was highly prevalent in extremely overweight adolescents meeting eligibility criteria for bariatric surgery. The significant weight loss after gastric bypass was associated with a marked reduction in OSA severity.