We studied the role of colour Doppler energy (CDE) (or power Doppler) imaging in the differentiation between endometriomas and other adnexal masses in premenopausal non-pregnant women. A total of 170 consecutive patients with persistent adnexal masses was submitted to B-mode transvaginal ultrasonography associated with CDE imaging evaluation. Plasma concentrations of CA125 were measured before surgery. Using CDE imaging evaluation of vessel distribution, the occurrence of one of the following findings was considered to indicate the likely presence of endometrioma: (i) a round-shaped homogeneous hypoechoic 'tissue' of low-level echoes without papillary proliferations associated with 'poor' vascularization; (ii) a round-shaped homogeneous hypoechoic 'tissue' of low-level echoes with an echogenic portion in which no flow was detected. The overall agreement between the test result and the actual outcome was calculated using the k index. The CDE imaging evaluation was more accurate in the diagnosis of endometriomas compared with B-mode ultrasonography alone (k = 0.88 and 0.80 respectively). According to the logistic regression equation obtained, the probability of the presence of endometrioma varied between a minimum of 1.4% for patients with no risk factors to a maximum of 95.6% for patients with two risk factors (CDE result and value of CA125 >25 IU/ml).