The polycystic ovary syndrome (PCOS) is frequently associated with obesity. However, there are very few data about PCOS in morbid obesity, especially with regard to its evolution after bariatric surgery. The objective of this study was to evaluate the response of PCOS to the sustained and marked weight loss achieved by bariatric surgery in morbidly obese women. This was a longitudinal prospective nonrandomized evaluation. The study was performed at an academic hospital. Thirty-six consecutive premenopausal women submitted to bariatric surgery were screened for PCOS, which was present in 17. Bariatric surgery was performed. Hyperandrogenism, menstrual function, and insulin resistance were estimated before and at least 6 months after bariatric surgery in 12 patients with PCOS. Weight loss (41 +/- 9 kg after 12 +/- 5 months) was paralleled by decreases in the hirsutism score (from 9.5 +/- 6.8 to 4.9 +/- 4.2; P = 0.001), total (69 +/- 32 to 42 +/- 19 ng/dl; P < 0.02) and free testosterone (from 1.6 +/- 0.7 to 0.6 +/- 0.3 ng/dl; P < 0.005), androstenedione (from 4.1 +/- 1.5 to 3.0 +/- 0.9 ng/ml; P < 0.02), and dehydroepiandrosterone sulfate (from 2000 +/- 1125 to 1353 +/- 759 ng/ml; P < 0.005); amelioration of insulin resistance estimated by homeostasis model assessment (from 6.0 +/- 3.0 to 1.6 +/- 1.0; P < 0.001); and restoration of regular menstrual cycles and/or ovulation in all patients. The PCOS is a frequent finding in women with morbid obesity and may resolve after weight loss induced by bariatric surgery.