Objective To quantify levels of macrophage migration inhibitory facto r (MIF) i n the peritoneal fluid (PF) of women with endometriosis, and to correlate these levels with the extent of disease. Design Controlled clinical study. Setting Aca demic medical center. Patient(s) Peritoneal fluid samples were collected during laparoscopic surgery in 60 women with endometriosis and 16 controls undergoing t ubal ligation; 52 of the women with endometriosis had received no hormonal treat ment in the 6 months prior to surgery, while 8 were using gonadotropin-releasin g hormone (GnRH) agonists. Main outcome measure(s) Peritoneal fluid migration in hibitory factor (PF MIF) levels. Result(s) Women with endometriosis had signific antly higher PF MIF levels (10.8 ±0.9 ng/mL) than controls (3.0 ±0.7 ng/mL). H owever, no correlation existed between MIF levels and the stage of disease (r = 0.05) or the depth of endometriotic invasion (r = 0.08). Moreover, treatment wit h a GnRH agonist did not suppress PF MIF levels. Peritoneal fluid MIF levels did not vary significantly between the proliferative and secretory phases of the cy cle, and did not distinguish women with endometriosis-associated infertility fr om women with endometriosis-associated pain. Conclusion(s) Peritoneal fluid mig ration inhibitory factor levels are markedly elevated in women with endometriosi s but are independent of the extent of disease. Objective To quantify levels of macrophage migration inhibitory facto r (MIF) in the peritoneal fluid (PF) of women with endometriosis, and to correlate these levels with the extent of disease. Design Controlled clinical study. Setting Aca demic medical center. Patient (s) ) Peritoneal fluid samples were collected during laparoscopic surgery in 60 women with endometriosis and 16 controls undergoing ubal ligation; 52 of the women with endometriosis had received no hormonal treat ment in the 6 months prior to surgery, while 8 were using gonadotropin-releasin g Results (s) Women with endometriosis had significantly higher PF MIF levels (10.8 ± 0.9 ng / mL) than controls (3.0) ± 0.7 ng / mL) H owever, no correlation existed between MIF levels and the stage of disease (r = 0.05) or the depth of endometriotic invasion (r = 0.08). Moreover, treatment wit ha GnRH agonist did not suppress PF MIF levels. Peritoneal fluid MIF levels did not vary significantly between the proliferative and secretory phases of the cy cle, and did not distinguish women with endometriosis-associated infertility fr om women with endometriosis-associated pain. Conclusion (s) Peritoneal fluid mig ration inhibitory factor levels are markedly elevated in women with endometriosi s but are independent of the extent of disease.