It has been widely recognized in the last decade that insulin resistance and hyperinsulinemia are inconsistently present in the majority of women with polycystic ovary syndrome (PCOS). In addition, obesity may affect more than half of these women. Obesity has a profound impact on the PCOS phenotype, being associated with a more severe insulin resistant state, hyperandrogenism and fertility disorders. Moreover, the association between obesity (particularly the abdominal phenotype) and PCOS renders affected women more susceptible to develop glucose intolerance and the metabolic syndrome in comparison with the general population. However, the presence of differences in the prevalence rate of glucose intolerance states or the metabolic syndrome within PCOS women in different countries suggests that environmental factors are important in determining individual susceptibility to develop these metabolic abnormalities.