BACKGROUND Adiponectin is a pleiotropic cytokine originally discovered as an adipocyte-specific gene product. Serum adiponectin concentrations have been reported to be low in women with endometrial cancer, breast cancer and uterine leiomyoma, suggesting possible involvement of adiponectin in these estrogen-related diseases. We thus addressed the relevance of adiponectin to endometriosis, an estrogen-dependent disease, in the present study. METHODS Women with (n = 48) and without (n = 30) endometriosis undergoing laparoscopy were recruited in this study. Blood samples were collected, and serum adiponectin concentrations were measured using a specific enzyme-linked immunosorbent assay. The relationship between laparoscopic findings and serum adiponectin concentrations was analysed. RESULTS The adiponectin concentrations in the serum of the women with endometriosis (median, 13.1 microg/ml; interquartile range, 10.2 - 16.7) were significantly lower than those of the women without endometriosis (15.9 microg/ml, 13.5 - 19.5; P = 0.008). A significant negative correlation was found between serum adiponectin concentrations and both endometriosis scores (R = - 0.307, P = 0.006) and adhesion scores (R = - 0.254, P = 0.026) of the revised American Society for Reproductive Medicine classification of endometriosis. CONCLUSIONS The present findings suggest that adiponectin is implicated in the pathophysiology of endometriosis.