Estrogens are involved in the immune response, and macrophages express estrogen receptors (ER). Moreover, macrophages are the predominant cell type in the peritoneal fluid from endometriosis patients. On this basis, the aim of our study was to evaluate the expression of ER on peritoneal macrophages from endometriosis patients and to compare these results with what is already known about ER and macrophages in RA. After macrophage extraction from peritoneal fluids we performed the immunohistochemical localization of ERalpha and ERbeta and then the image analysis. We found that both ERs were significantly overexpressed in macrophages of women with endometriosis compared with controls. These results suggest that estrogens, through their functional receptors, might modulate the immune response at least on macrophages. Therefore, estrogens seem to play an important role in the immune response, independently from the pathology.