The benign cystic ovarian teratoma (dermoid) has distinctive ultrasonic features that differentiate it from other adnexal masses. The most frequent sign is the presence of highly reflective irregular solid components within a fluid-containing adnexal mass. In vitro scans of surgical specimens have shown that the strongly reflective echo pattern is caused by the hair and sebum within the dermoid. Calcium is often sparse or absent. Acoustic shadowing from the hair may totally obscure the back wall of a large, clinically evident mass; hence the term "tip of the iceberg" sign. Recognition of this sign reduces the possibility of a false negative interpretation and confusion with a "bowel gas pattern."