It has been recently reported that serum concentrations of the adipocyte-derived hormone leptin are increased in patients affected by endometriosis. On the basis of these findings, the present study was undertaken to evaluate whether the protein may be used as a new serum marker of the disease. A consecutive series of 67 reproductive-age women who underwent laparoscopy for benign gynecological pathologies were enrolled prospectively for the study. Serum leptin concentrations, as evaluated by a conventional RIA kit, were related to baseline clinical characteristics and surgical and histologic diagnosis. Endometriosis was documented in 42 women (stage I-II in 19 patients and stage III-IV in 23 patients). Twenty-five women of similar age and body mass index, who had no laparoscopic evidence of the disease, served as control group. Serum levels of leptin resulted similar between women without and with endometriosis at any stage (mean +/- SEM, 12.5 +/- 9.4 ng/ml and 12.1 +/- 8.0 ng/ml, respectively). No significant association with leptin concentrations was observed in regard to stage of the disease, number of endometriotic implants, presence/absence of an endometriotic cyst or peritoneal deep endometriosis, and presence/absence of specific symptoms. Therefore, our results do not support the possibility to employ leptin measurement as a diagnostic tool for endometriosis. Further studies are needed to elucidate the relationship between leptin and endometrial system and determine the potential contribution of the molecule in implantation and early pregnancy development.