Animal and human studies suggest that leptin modulates hypothalamic-pituitary-gonadal axis functions. Leptin may stimulate gonadotrophin-releasing hormone (GnRH) release from the hypothalamus and luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion from the pituitary. A synchronicity of LH and leptin pulses has been described in healthy women, suggesting that leptin probably also regulates the episodic secretion of LH. In some pathological conditions, such as polycystic ovarian syndrome (PCOS), LH-leptin interactions are not known. The aim of the present investigation was to assess the episodic fluctuations of circulating LH and leptin in PCOS patients compared to regularly menstruating women. Six PCOS patients and six normal cycling (NC) women of similar age and body mass index (BMI) were studied. To assess episodic hormone secretion, blood samples were collected at 10-min intervals for 6 h. LH and leptin concentrations were measured in all samples. For pulse analysis the cluster algorithm was used. To detect an interaction between LH and leptin pulses, an analysis of copulsatility was employed. LH concentrations were significantly higher in the PCOS group in comparison to NC women, however serum leptin concentrations and leptin pulse characteristics for PCOS patients did not differ from NC women. A strong synchronicity between LH and leptin pulses was observed in NC women; 11 coincident leptin pulses were counted with a phase shift of 0 min (P = 0.027), 18 pulses with a phase shift of -1 (P = 0.025) and 24 pulses with a phase shift of -2 (P = 0.028). PCOS patients also exhibited a synchronicity between LH and leptin pulses but weaker (only 20 of 39 pulses) and with a phase shift greater than in normal women, leptin pulses preceding LH pulses by 20 min (P = 0.0163). These results demonstrate that circulating leptin and LH are synchronized in normal women and patients with PCOS. The real significance of the apparent copulsatility between LH and leptin must be elucidated, as well as the mechanisms that account for the ultradian leptin release.