Overweight and obesity are common findings in polycystic ovary syndrome (PCOS). Obesity-particularly central obesity-is strongly indicated as a cause of insulin resistance, a central feature of PCOS. The prevalence of obesity is reaching epidemic proportions in many developed countries, and this increase is of particular concern in adolescent women. Obesity worsens both the metabolic and endocrine profiles in PCOS and may decrease the response to treatment. In the short term, weight reduction improves both metabolic and endocrine aspects of PCOS as well as clinical markers such as ovulation. However, extreme non-surgical efforts to lose weight are rarely successful and are associated with high rates of weight regain. Lifestyle modification with modest weight loss goals of 5-10% appear to be equally effective in restoring fertility and may be more compatible with long-term success; however, further research is needed.