PURPOSE: To evaluate whether CT scanning is useful in differentiating the between endometriomas and other benign complex cystic adnexal masses, and in determining the method of treatment for each mass lesion. MATERIALS AND METHODS: In 54 cases (47 patients), we retrospectively analysed the CT findings of 20 pathologically-proven twenty endometriomas (bilateral in four cases), eight hemorrhagic functional cysts, two tubal ectopic pregnancies, eight tubo-ovarian abscesses (bilateral in two cases), ten serous cystadenomas (bilateral in one case), and six mucinous cystadenomas. Internal attenuation, the hyperdense portion, adhesion, and cul-de-sac obliteration were evaluated by CT scanning. RESULTS: Fourteen endometriomas (70%) showed a hyperdense portion, and in only two of these (10%), was a focal nodular hyperdense portion seen on pre-contrast CT scan (10% sensitivity, 100% specificity). Partial or complete cul-de-sac obliteration was identified in 11 patients (75%), while hemorrhagic functional cysts showed a hyperdense portion in four cases (50%) and were accompanied by partial cul-de-sac obliteration in two (25%). Two unruptured tubal ectopic pregnancies showed CT findings of unilateral hyperdense cystic masses of more than 60 HU. In all cases, tubo-ovarian abscesses were accompanied by thickening of the uterosacral ligament and deviation of thickened mesosalpinx (anterior deviation in 87.5% of patients). Serous and mucinous cystadenomas showed CT findings of hypodense masses (less than 20 HU) without adhesion or cul-de-sac obliteration, and this was helpful in differentiating cystadenomas from other benign cystic adnexal masses, including endometriomas. CONCLUSION: The evaluation by CT scanning of benign complex cystic adnexal masses with respect to the hyperdense portion and the presence or absence of cul-de-sac obliteration was usful in differentiating endometriomas from other lesions, and might be helpful in determining the method of treatment for each mass lesion.