Antiendometrial antibodies were tested by passive hemagglutination assay(PHA) and enzyme linked immunosorbent assay(ELISA) in sera of 45 patients with endometriosis and 21 controls for the evaluation of its clinical usefulness. Both titers(reciprocal) of serum antiendometrial antibody by PHA and ELISA reactivity against endometrial antigens at 405nm were significantly higher in patients with endometriosis than in the control group. Of 45 patients with endometriosis, twenty-six(57.8%) by PHA and twenty-three(51.1%) by ELISA were determined antiendometrial antibody positive but none of the controls were positive. Antiendometrial antibody titers were followed up by PHA and ELISA in 14 patients with high antiendometrial antibody titers before and after treatment with Decapeptyl. There was a decreasing tendency of antiendometrial antibody titers after treatment but there was no statistical significance. These data suggest that the measurement of circulating antiendometrial antibody may be useful for the diagnosis but not for the follow up of endometriosis.