Women with polycystic ovary syndrome have an increased rate of obesity, with a propensity toward abdominal deposition of body fat. Independent of obesity, at least half of affected women have insulin resistance. To understand the mechanisms of insulin resistance in polycystic ovary syndrome, it is necessary to understand normal insulin signaling. Women with polycystic ovary syndrome have normal binding of insulin to its receptor but have decreased activation of events downstream of the receptor. This insulin resistance occurs mostly in the peripheral tissues (muscle and fat cells), and results in increased pancreatic insulin secretion to maintain normal glucose levels. Obese women with polycystic ovary syndrome have a relative defect in insulin secretion. At least 20% of obese patients with polycystic ovary syndrome have glucose intolerance or diabetes, versus about 5% of the healthy age-matched population. Obesity and insulin resistance probably increase the risk of cardiovascular disease in women with polycystic ovary syndrome. The metabolic features of polycystic ovary syndrome are important health risk factors and need to be considered seriously, even if the patient seeks treatment for other concerns.