Evidence for a relationship between melatonin and the reproductive hormones in humans is based on observations of abnormal melatonin secretion in clinical disorders of the pituitary-gonadal axis. The aim of this study was to investigate melatonin production in hyperandrogenic women before and during treatment with cyproterone acetate and ethinyl estradiol (Diane 35). Twelve women with polycystic ovary syndrome (PCOS), 10 women with idiopathic hirsutism (IH), and 10 women with late onset adrenal hyperplasia due to 21-hydroxylase deficiency (LOCAH) were studied. Patients were treated with Diane 35 for four months. Fasting blood samples for the determination of luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone and dihydroepiandrosterone sulfate (DHEAS) and 24-hour urine collections for the determination of 6-sulfatoxymelatonin (aMT6s) excretion were obtained from all patients at baseline and after 4 months of treatment. Results were compared with those obtained in 15 control women. At baseline, women with PCOS and LOCAH had significantly higher testosterone and aMT6s values than women with IH and controls. Diane 35 treatment significantly decreased testosterone, LH, FSH and aMT6s values in PCOS and LOCAH patients compared with pretreatment values. These results indicate that hyperandrogenic women with PCOS and LOCAH have increased melatonin production. The decrease in aMT6s excretion together with reduced serum LH, FSH, DHEAS and testosterone values during treatment with cyproterone acetate-ethinyl estradiol, suggest that sex steroids either directly or through the suppression of gonadotropins, modulate melatonin secretion in these patients.