Polycystic ovary syndrome (PCOS) is a heterogeneous syndrome determined in most patients by the association of two main factors: hyperandrogenism and insulin resistance. These characters are probably independent of each other and seem to be inherited by several different mechanisms. In some patients homozygous gene alteration has been found but in most patients PCOS seems to be determined by the association of gene polymorphisms that are common in the general population but alone are unable to determine phenotypic consequences. Alteration of genes that regulate the initial steps of ovarian steroidogenesis is probably the main causal factor of hyperandrogenism. Insulin resistance may be the result of many different gene alterations including insulin receptor substrate (IRS)-1 and 2, calpain-10 and peroxisome proliferator-activated receptor gamma(PPARgamma). Some polymorphisms may be protective against other gene alterations. Insulin sensitivity is also modified by socioeconomic and cultural factors that influence quantity and quality of food and energy expenditure. However, even eating behavior and weight response to food intake may be under genetic regulation. Different combinations of multiple gene polymorphisms and of environmental factors explain the heterogeneity of PCOS.