['To investigate menstrual cycle changes in expression by eutopic endometrium of a nonclassic human leukocyte antigen, HLA-G, which binds to the killer immunoglobulin-like receptor (KIR) 2DL4 (CD158d) on natural killer (NK) cells. Such antigens have been linked to endometriosis.', 'Case-control study.', 'University hospital.', 'We examined 20 Japanese women undergoing hysterectomy for endometriosis and 17 undergoing hysterectomy for myoma.', 'Immunohistochemical HLA-G staining of eutopic endometrium and peritoneal fluid (PF) cells from women with and without endometriosis. Flow cytometric analysis of PF NK cells from women with and without endometriosis.', 'HLA-G staining in eutopic endometrium was quantified by image analysis. The KIR2DL4-expressing NK cells in PF were investigated by flow cytometry.', 'The HLA-G was expressed by eutopic endometrium only in the menstrual phase and not in the late proliferative or secretory endometrium. Intensity of HLA-G staining did not differ significantly between women with and without endometriosis. The HLA-G- expressing cells were detected in PF during the menstrual period. These cells are morphologically and flow cytometrically different from mesothelial cells and NK cells.'] Is human leukocyte antigen-G , a ligand for the natural killer receptor KIR2DL4 , expressed by eutopic endometrium only in the menstrual phase?