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 RouxenY gastric bypass and again 1year 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 42years (2262years), 127kg (87176kg), and 46.8kg/m2 (3664.3kg/m2), 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 42kg (1967kg). Simple hyperplasia was identified in 3/46 women at the 1year 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.