Approximately half the women with the polycystic ovary syndrome (PCOS) are obese or overweight. Obesity and body fat distribution have independent roles in the development of hyperandrogenism in PCOS. Most obese and normal weight PCOS are insulin resistant and hyperinsulinemic. Moreover, a significant positive correlation exists between the degree of hyperandrogenism and that of hyperinsulinism. The pathogenetic role of obesity may involve different mechanisms, the major one being the hyperinsulemic state, since insulin is capable of stimulating ovarian androgen secretion and controlling androgen metabolism and transport in peripheral tissues. Abdominal body fat distribution in obese women with PCOS amplifies the degree of hyperandrogenism and related clinical symptoms and signs. Both loss of body weight and/or the reduction of the degree of hyperinsuliemia, induced by diet or insulin-sensitizing drugs, have important effects, since they reduce blood androgen levels and can improve ovulation and clinical signs of hyperandrogenism.