It is well known that a strong exponential relationship exists between leptin levels and body mass index (BMI). The different serum leptin levels, however, that are observed for each BMI value, suggest that other factors, as well, interfere with leptin secretion. This study was designed in order to estimate serum leptin levels in patients with polycystic ovary syndrome (PCOS), before and after long-term treatment with conjugated estrogens and antiandrogens. Sixteen women with PCOS were included in the study. They were divided into two groups: the first group comprised 11 non-obese women (BMI 21.6 +/- 0.5 kg/m2), aged 23.5 +/- 1.1 years; the second consisted of five obese women (BMI 28.9 +/- 1.5 kg/m2), aged 22.8 +/- 1.9 years. Blood samples for leptin measurement were drawn before and 2, 4, 6, 9 and 12 months after administration of conjugated estrogens and antiandrogens. Our results showed that obese women exhibited higher serum leptin levels in all blood samples. Moreover, the administration of conjugated estrogens and antiandrogens caused an increase in serum leptin levels in the 2nd, 4th, 6th and 9th month in both groups of women. Finally, leptin concentrations during the 12th month of the treatment returned to basic levels in both groups of women with PCOS. Our results support the view that BMI is the main variable that influences serum leptin levels, and that the effect of conjugated estrogens and antiandrogens on serum leptin concentrations is poor.