The incidence of obstructive sleep apnea has been underestimated in morbidly obese patients who present for evaluation for weight loss surgery. This retrospective study shows that the incidence of obstructive sleep apnea in this patient population is greater than 70 per cent and increases in incidence as the body mass index increases. Obstructive sleep apnea (OSA) is a common comorbidity in obese patients who present for evaluation for gastric bypass surgery. The incidence of sleep apnea in obese patients has been reported to be as high as 40 per cent. A retrospective review of our prospectively collected database was performed. All patients being evaluated for weight loss surgery for obesity were screened preoperatively for OSA using a sleep study. The overall incidence of sleep apnea in our patients was 78 per cent (227 of 290). All 227 were diagnosed by formal sleep study. There were 63 (22%) males and 227 (78%) females. The mean age was 43 years (range, 17-75 years). The mean body mass index (BMI) was 52 kg/m2 (range, 31-94 kg/m2). The prevalence of OSA in the severely obese group (BMI 35-39.9 kg/m2) was 71 per cent. For the morbidly obese group (BMI 40-40.9 kg/m2), the prevalence was 74 per cent and for the superobese group (BMI 50-59.9 kg/m2) 77 per cent. Those with a BMI 60 kg/m2 or greater, the prevalence of OSA rose to 95 per cent. The incidence of sleep apnea in patients presenting for weight loss surgery was greater than 70 per cent in our study. Patients presenting for weight loss surgery should undergo a formal sleep study to diagnose OSA before bariatric surgery.