The aims of the present study were to identify the cause of hyperprolactinemia in polycystic ovary syndrome (PCOS) and to compare prolactin (PRL) levels between PCOS women without hyperprolactinemia and women with insulin resistance and without PCOS. A group of 82 women (age: 27.1 +/- 7.6 years) with PCOS was included in the study. Their PRL levels were measured and compared with those of women with insulin resistance without PCOS (controls; n = 42; age: 29.2 +/- 8.2 years). Among the 82 PCOS women, 13 (16%) presented high PRL levels (103.9 +/- 136.0 microg/l). The causes of hyperprolactinemia were: pituitary tumor (responding to cabergoline) in nine cases (69%; PRL range: 28.6 - 538 microg/l); oral hormonal contraceptive treatment in two cases (15%; PRL: 46 and 55 microg/l, respectively); and use of buspirone and tianeptine in one case (8%; PRL: 37.1 microg/l); one case (8%; PRL: 34.4 microg/l) had macroprolactinemia. In drug-induced hyperprolactinemic patients PRL levels normalized after treatment interruption. The average PRL level in the 69 remaining patients was 12.1 +/- 5.5 microg/l, a value not statistically different from that of the control group (11.8 +/- 4.9 microg/l). This result leads us to conclude that PCOS patients with increased PRL levels must be investigated for other causes of hyperprolactinemia, because hyperprolactinemia is not a clinical manifestation of PCOS.