AbstractObjectivesTo describe the ultrasound characteristics of endometriomas in pre‐ and postmenopausal patients and to develop rules that characterize endometriomas.MethodsAll patients included in the International Ovarian Tumor Analysis (IOTA) studies were used in our analysis. Patients with an adnexal mass were scanned by experienced sonologists using a standardized research protocol. The gold standard was the histology of the surgically removed adnexal mass. The gray‐scale and Doppler ultrasound characteristics of the endometriomas were compared with those of other benign and malignant masses. Based on decision‐tree analysis, the existing literature and clinical experience, ultrasound rules for the detection of endometriomas were created and evaluated.ResultsOf all 3511 patients included in the IOTA studies, 713 (20%) had endometriomas. Fifty‐one per cent of the endometriomas were unilocular cysts with ground glass echogenicity of the cyst fluid. These characteristics were found less often among other benign tumors or malignancies, or among the small set of endometriomas (4%) that were found in postmenopausal patients. Based on the decision‐tree analysis, the optimal rule to detect endometriomas was ‘an adnexal mass in a premenopausal patient with ground glass echogenicity of the cyst fluid, one to four locules and no papillations with detectable blood flow’. Based on clinical considerations, the following rule: ‘premenopausal status, ground glass echogenicity of the cyst fluid, one to four locules and no solid parts’ seems preferable.ConclusionsSeveral rules had a good ability to characterize endometriomas. The ultrasound characteristics of endometriomas differ between pre‐ and postmenopausal patients. Masses in postmenopausal women whose cystic contents have a ground glass appearance have a high risk of malignancy. Copyright © 2010 ISUOG. Published by John Wiley & Sons, Ltd.