Endometriosis, a debilitating disease associated with infertility, is characterized by the prolonged presence of ectopic endometrial tissue and the involvement of activated peritoneal fluid macrophages. Apoptosis, which occurs in both endometrium and peritoneal fluid macrophages, is controlled in part by members of the Bcl-2/Bax family of proteins. Here, through immunohistochemical staining, we investigated the Bcl-2/Bax status in endometrium and peritoneal fluid macrophages in endometriosis. Bcl-2/Bax immunoreactivity was found predominantly in the glandular epithelial cells, mainly during the proliferative phase of the menstrual cycle for Bcl-2 but throughout the entire menstrual cycle for Bax. Ectopic endometrium contained a population of Bcl-2 positive. Bax negative tissue macrophages. Fluorescence-activated cell sorting of isolated peritoneal fluid macrophages showed that women with endometriosis had a significantly higher proportion of Bcl-2 positive macrophages than the non-endometriotic group. The proportion of Bax positive peritoneal fluid macrophages was significantly elevated in women without endometriosis. The increased proportion of Bcl-2 positive macrophages found in women with endometriosis may predispose these cells to resist apoptosis. The continued survival of these active cells could have important consequences for the survival and proliferation of the ectopic endometrial tissue.