OBJECTIVE: To determine the prevalence of occult uterine pathology in asymptomatic, morbidly obese women before and after bariatric surgery-induced weight loss. DESIGN: Prospective, blinded, non-interventional cohort. SETTING: Urban teaching hospital. POPULATION: Morbidly obese women. METHODS: Endometrial biopsies were obtained at the time of Roux-en-Y gastric bypass and again 1 year later. Both the patient and the physician were blinded to the results of the initial biopsy until the conclusion of the study. Specimens were independently reviewed by two blinded pathologists. MAIN OUTCOME MEASURE: Effect of bariatric surgery-induced weight loss on the prevalence of endometrial pathology at 1 year. RESULTS: Fifty-nine women underwent an endometrial biopsy during bariatric surgery. The mean (range) age, weight, and body mass index (BMI) were 42 years (22-62 years), 127 kg (87-176 kg), and 46.8 kg/m(2) (36-64.3 kg/m(2) ), respectively. Four women had hyperplasia (three simple and one complex), for an overall prevalence of 6.8%. The prevalence among women not receiving some anti-estrogen therapy was 9.5%. Forty-six women (78%) underwent follow-up biopsy after a mean (range) weight loss of 42 kg (19-67 kg). Simple hyperplasia was identified in 3/46 women at the 1-year follow-up (6.5%). Two women had resolution of hyperplasia, two women had persistent, simple hyperplasia, and one had had a normal initial biopsy. No woman showed progressive pathology or cancer. At the end of the follow-up all but one patient had a documented resolution of endometrial pathology. CONCLUSIONS: Asymptomatic morbidly obese women are at relatively high risk of harbouring occult endometrial hyperplasia. Bariatric surgery-associated weight loss reduced but did not eliminate this risk for endometrial pathology.