Obesity, ultrasonic ovarian morphology, serum LH levels and LH/FSH ratio are inconstant symptoms of the polycystic ovary syndrome (PCOS) and are thus no longer essential for diagnosis. PCOS is diagnosed today by the finding of chronic anovulation and hyperandrogenism characterized by a high serum level of "free" testoterone. The other causes of hyperandrogenism, as well as anovulations due to hyperprolactinemia, high levels of FSH and abnormal thyroid function have to be ruled out. PCOS is very often associated with insulin resistance (IR) and hyperinsulinemia (hyper I). From in vitro and vivo studies and treatment of hyper I, it has been shown that the hyper I of PCOS stimulates androgen production. Hyper I of PCOS increases the activity of androgens: by first provoking an important decrease of the sex hormone binding globulin (SHBG) thus increasing the "free", bioactive testosterone level. and then by activating the cytochrome P 450 c 17 alpha enzymatic system that controls androgen production. Subsequent to metformin administration, the reduction of hyper I and androgen serum levels creates a favorable condition for the resumption of ovarian function and clomiphene citrate action. This explains the high percentage of ovulations and pregnancies.