Endometriosis is a common, complex and chronic disease related to ectopic implantation and growth of endometrial tissue that may manifest by pelvic pain, and accounts for over 20% of all cases of infertility in women. Endometriosis may be associated with increased levels of leptin in peritoneal fluid. However, the association of leptin with infertility has not been definitely documented. Therefore, the aim of the present study was to search for a relationship between concentrations of peritoneal-fluid leptin and patients' clinical status. The study included 56 patients being diagnosed for infertility and/or pelvic pain. Peritoneal fluid was aspirated during routine laparoscopic examination. Concentrations of leptin in peritoneal fluid were evaluated by a specific enzyme-linked immunosorbent assay. The results revealed that the levels of peritoneal-fluid leptin did not correlate with different stage of endometriosis. However, they correlated with body mass index. Leptin levels were significantly higher in infertile patients than in patients with pelvic pain (p = 0.0023 by Mann-Whitney U test or p = 0.0045 by analysis of variance). It may suggest that increased leptin levels in the peritoneal fluid may play a role in pathogenesis of infertility.