The clinical heterogeneity of the syndrome is the product of a multifaceted process; where converge genetic and environmental influences. Many hypotheses try to explain the primary defect; but insulin resistance (IR) seems the most suitable according to most of studies. IR is present in 60 to 70 % of patients independently of obesity. Compensatory hyperinsulinism has a fundamental role in the pathology of PCOS. In vitro insulin stimulates androgen synthesis in ovary theca cells, acting on its own receptor and inhibits hepatic synthesis of sex hormone binding globulin (SHBG), increasing free testosterone. It also leads to the amplification of LH induced expression of cytochrome P450c 17 alfa (a limiting enzyme in androgen synthesis).