Obesity may be an important pathogenetic factor involved in the development of hyper-androgenism in women with polycystic ovary syndrome (PCOS). Among several other mechanisms, hyperinsulinaemia plays a fundamental role, due to its gonadotrophic function, which has been demonstrated both in vitro and in vivo. Therefore, not surprisingly, weight loss may be expected to have several beneficial effects upon clinical, endocrinological and metabolic features of obese women presenting both PCOS. In particular, weight loss appears to be associated with a significant improvement in menses abnormalities, ovulation and fertility rates, and with a reduction of hyperandrogenism, hyperinsulinaemia, and altered gonadotrophin pulsatile secretion. The central role of improved insulin concentrations and insulin-resistant state is emphasized by the fact that similar effects can be achieved by both short- and long-term administration of metformin, an insulin-lowering drug which ameliorates peripheral insulin action in non-diabetic insulin resistant states. We therefore recommend weight loss as a first-line therapeutic option in all women with obesity and PCOS.