BACKGROUND Adiponectin is regarded as a possible link between adiposity and insulin resistance. The study aim was to measure serum adiponectin levels in women with polycystic ovary syndrome (PCOS) and to assess possible correlations between adiponectin and the hormonal or metabolic parameters of the syndrome. METHODS Eighty-five selected women were classified as: Group I (n = 35) with PCOS + body mass index (BMI) >25 kg/m(2); group II (n = 35) with PCOS + BMI <25 kg/m(2); and group III (controls; n = 15) ovulating without hyperandrogenaemia and BMI <25 kg/m(2). Blood samples were collected between the days 3 and 6 of a spontaneous menstrual cycle, at 09:00, after an overnight fast. Serum levels of FSH, LH, prolactin, 17alpha-OH-progesterone, sex hormone-binding globulin (SHBG), androgens, insulin, adiponectin and glucose were measured. RESULTS Adiponectin levels were significantly decreased in group I compared with groups II and III. No significant difference in adiponectin levels was found between groups II and III, despite significant differences in insulin levels and glucose:insulin ratio. Multiple regression analysis showed that Delta(4)-androstenedione levels and BMI values were the only significant determinants of serum adiponectin levels. CONCLUSIONS Serum adiponectin levels are reduced in obese women with PCOS. Although adiponectin does not seem to be actively involved in the pathogenesis of PCOS, there seems to be an interaction between adiponectin and steroid synthesis or action.