Objectives: Endometriosis, a common benign gynecological disease, is characterized by the presence and growth of endometrial tissue outside the uterus. The widely accepted hypothesis regarding its etiology is implantation of endometrial cells inside the peritoneal cavity during retrograde menstruation. We hypothesized that apoptosis, an essential cell death mechanism responsible for maintaining tissue homeostasis, may play a role in the implantation, survival, and progression of endometriotic tissue. The purpose of this study was to investigate apoptosis and expression of cell death-associated genes in endometriosis.Design: Apoptosis was detected by in situ DNA end-labeling and autoradiographic analysis of internucleosomal DNA cleavage in matched ectopic and eutopic endometrial tissues, as well as endometria from women without endometriosis. Expression of members of the BCL-2 family of cell death regulatory proteins, including BCL-2, BAX, BCL-X and BAK, were evaluated by immunohistochemistry and Western blot analysis. In general, BCL-2 and BCL-X l o n g inhibit apoptosis, whereas BAX, BCL-X s h o r t , and BAK promote cell death.Materials and Methods: Matched ectopic and eutopic endometrial biopsies were obtained simultaneously from 4 patients with documented endometriosis at the time of laparoscopy. Another two normal endometrial biopsies were obtained from women without endometriosis. All tissues were obtained during the proliferative phase between days 5 and 10 of the menstrual cycle. Samples were frozen and used for extracting protein and DNA, or for preparing frozen sections for histochemical manipulations.Results: By autoradiographic analysis of DNA cleavage, there was a higher rate of apoptosis in both ectopic and eutopic endometrial tissues from women with endometriosis compared to endometrium from women without endometriosis. In women with endometriosis, in situ DNA labeling demonstrated scattered apoptotic cells in both stroma and glands in ectopic and eutopic endometrium, with increased apoptosis in ectopic tissue. By comparison, apoptosis was generally less in women without endometriosis and was almost exclusively limited to glandular epithelial cells. Immunoreactive staining for BCL-2 was noted in most glandular epithelial cells and some stromal cells in both eutopic and ectopic endometrial tissues. BCL-X (predominantly by BCL-X l o n g by Western blot analysis) was concentrated in glandular epithelial cells in all samples. As compared with normal endometrium, immunoreactivity of BAK and BAX was higher in both ectopic and eutopic endometrium from patients with endometriosis. BAX was located primarily in glandular epithelial cells and BAK was distributed in both glandular epithelial and stromal cells, with stronger staining in endometriotic tissue.Conclusion: Our data suggest a differential pattern of apoptosis and expression of members of BCL-2 family in endometriosis, which may underlie the pathophysiology of endometriosis.