Context Lifestyle intervention is the first line treatment in Polycystic Ovary Syndrome (PCOS). However, to our knowledge there are no previous randomized trials comparing lifestyle intervention with no or minimal intervention having reproductive function as a primary outcome. Objective To evaluate the effect of a structured behavioral modification intervention on reproductive outcomes in overweight/obese women with PCOS. Methods In a randomized controlled trial patients were allocated to behavioral modification intervention or minimal intervention in a 1:1 ratio for comparisons after 4 months treatment. The minimal intervention group received behavioral modification intervention for a further 4 months. All patients were evaluated at 12 months in comparison to baseline. Patients 68 women aged 18-40 years, with a BMI ≥ 27 kg/m2, fulfilling all 3 Rotterdam PCOS criteria were included. Intervention Behavioral modification intervention consisted of a structured approach aiming to achieve weight loss in order to improve reproductive function. This included regular individual and group meetings with a lifestyle coach and dietician. Minimal intervention involved general oral and written health advice from a midwife. Main Outcome Measures The primary reproductive outcome was improved menstrual regularity defined as a change from amenorrhea to either oligomenorrhea or regular cycles, or from oligomenorrhea to regular cycles during the first 4 months. Secondary reproductive outcome was ovulation, determined by progesterone levels. Results The mean weight loss at 4 months compared to baseline for patients in the behavioral modification group was 2.0 kg, P = .002 and for the minimal intervention group 0.99 kg P=.128. At 12 months the mean weight loss for the whole group was 3.0 kg, P = .003 compared to baseline. At 4 months, a higher proportion of patients in the behavioral modification group improved menstrual function compared to the minimal intervention group, 59% vs 24%, P = .003. At 12 months, a total of 54% improved their menstrual function. No difference in ovulation was seen between the groups (35% vs 34%) but during the 12 months study period, 53% of all patients had ovulated. Conclusion Although extensive weight loss can be difficult to achieve in obese women with PCOS, behavioral modification intervention can help these patients to improve their menstrual regularity and ovulation, and subsequently improve the chances of spontaneously conceiving.