Impaired natural killer (NK) activity in women with endometriosis is thought to promote implantation and progression of endometrial tissue, in accord with Sampson's hypothesis. However, the mechanisms responsible for decreased NK cell activity and the antigens recognized by NK cells are not clear. We focused on human leukocyte antigen (HLA)-G, a ligand of NK receptors, expression and its menstrual cycle changes by eutopic endometrium. Interestingly, HLA-G expression was identified on eutopic endometrium only in the menstrual phase but not in the proliferative or secretory phases. Furthermore, HLA-G expressing cells were also detected in peritoneal fluid during the menstrual period. During retrograde menstruation, HLA-G expressing endometrial tissue may enter the peritoneal cavity, and may be reduced by immunosurveillance system. Although peritoneal NK cells play an important role in this system, impairment of NK cytotoxicity via HLA-G may allow peritoneal endometrial cell survival and implantation. In this review, we discuss the pathogenesis of endometriosis from the viewpoint of intraperitoneal immune interaction between NK cell receptors and HLA-G that can enter into peritoneal cavity from eutopic endometrium through retrograde menstruation.