PURPOSE: To analyze the sonographic, morphologic, and internal echo patterns of endometriomas, and thus determine which ultrasonographic (US) findings assist diagnosis. MATERIALS AND METHODS: One hundred and forty-seven eases of pathologically proven adnexal masses in 130 women were divided into three groups: group I, in which endometriomas were diagnosed at both preoperative US and surgery (true positive) (n=97); group II, in which endometriomas were misdiagnosed at preoperative US, and were confirmed after surgery to be other pathologic entities (false positive) (n=10); group III, in which other adnexal masses were misdiagnosed at preoperative US, but were proven after surgery to be endometriomas (false negative) (n=40). The US findings in these cases were retrospectively reviewed in terms of (a) morphologic type: unilocular, multiseptated, multilobulated, solid and cystic, or mixed; (b) internal echo pattern: homogeneous fine, anechoic, fine septation, or complex; (c) size; (d) wall thickness; (e) the presence or absence of septation; (f) wall nodularity; (g) echogenic wall foci; and (h) a solid area. RESULTS: In group I, the most common morphological type was unilocular cyst (n=63; 65%). In lesions most commonly emitted homogeneous fine echoes (n=76; 78%). In this group, most masses (86%) were less than 10 cm in diameter and the wall thickness in 65% of cases was less than 3 mm. Additionally, internal septation, wall nodularity, focal echogenic wall foci, and a solid area were observed at US. Group II, cases were pathologically confirmed as mucinous cystadenoma (n=3), mucinous cystadenoma with borderline malignancy, hemorrhagic cyst, functional cyst, endometrioid carcinoma, and hematoma. In group III, cases were misdiagnosed as cystadenoma (n=15), hemorrhagic cyst, teratoma, ovarian cancer, functional cyst and ectopic pregnancy at preoperative US. There were no significant differences in size or wall thickness between groups II and III, and group I. At US, groups II and III also showed internal septation, wall nodularity, focal echogenic wall foci, and a solid area, all of which were also apparent in group I. CONCLUSION: The US findings of endometriomas vary: the most common is homogeneous fine internal echoes (79%), found in 85% of unilocular or multiseptated cysts. Their appearance may also be atypical, however: namely solid and cystic or mixed type, with diverse internal echogenicity, and such masses should be differentiated from other adnexal masses such as cystic neoplasm, teratoma, hemorrhagic cyst, functional cyst and ovarian cancer.