Objective : Our purpose of study was to investigate whether in vivo levels of tumor necrosis factor-α in plasma and peritoneal fluid (PF) differ in women with and without endometriosis. Design : Prospective and case-control study. Methods : Fifty-seven women with laparotomy or laparoscopic findings of minimal to severe endometriosis, and forty-two women with no visual evidence of pelvic endometriosis and with benign gynecologic disease. Tumor necrosis factor-α levels in plasma and PF were determined using commercial ELISA. Tumor necrosis factor-α concentrations were compared among women with and without endometriosis, and then also were compared according to the revised American Fertility Society classification. Results : Tumor necrosis factor-α concentrations were not significantly increased in the plasma and PF of women with endometriosis than in matched normal controls. Tumor necrosis factor-α concentrations in endometriosis stage III and IV were sightly increased, which were not increased statistically significant. Conclusions : Plasma and PF tumor necrosis factor-α levels were not different between women with and without endometriosis. Our results do not rule out the hypothesis that tumor necrosis factor-α may be involved in the pathogenesis of some features of endometriosis.