FUKAYA, T., SUGAWARA, J., YOSHIDA, H. and YAJIMA, A. The Role of Macrophage Colony Stimulating Factor in the Peritoneal Fluid in Infertile Patients with Endometriosis. Tohoku J. Exp. Med., 1994, 172 (3), 221-226-Macrophage colony stimulating factor (M-CSF) concentrations in the peritoneal fluid of 44 infertile patients were measured. The concentration of M-CSF was significantly higher in the patients with endometriosis than in those without endometriosis. There was no significant difference between the M-CSF concentration and stage of endometriosis classified by revised American Fertility Society (rAFS) classification in the 44 patients. However, in primary infertility patients with endometriosis there was a significant correlation between peritoneal M-CSF levels and rAFS points of endometriosis. We speculate that there is some relationship between the increased levels of M-CSF in the peritoneal cavity and the primary infertility associated with endometriosis especially progression of endometriosis.