Polycystic ovary syndrome (PCOS) is a frequent disease, characterized by disturbed ovarian function with hyperandrogenism. Anovulation is secondary to an absence of follicular dominance. Apart from a primary ovarian defect, insulin resistance is observed in PCOS women, even in the absence of overweight. This insulin resistance could be secondary to a defect in the insulin transduction pathway, mainly by a defect in receptor phosphorylation. It enhances hyperandrogenism as it increases ovarian androgen production. Therefore treating insulin resistance by weight loss or drugs reducing insulin resistance might improve fertility of PCOS women. Metformin has been shown to reduce ovarian production, enhance ovulatory cycles and in some cases increase fertility. However, there are few randomized studies on large numbers of patients to prove an effect on pregnancies as well as on the occurrence of early pregnancy loss. There are currently no recommendation on dose and duration of metformin treatment. It is noteworthy that metformin has no authorization in France to be prescribed apart from diabetic patients' care. Considering the medical care of PCOS women, the cardiovascular risk needs to be taken into account. Therefore hypertension, dyslipidemia and diabetes must be treated in those women who need to be followed carefully all over their life.