In order to determine whether sonography could differentiate between benign and malignant ovarian neoplasms a retrospective analysis of preoperative ultrasound examination was made. The ultrasound images were evaluated for internal consistency, presence of septae, presence of solid nodules, papillary projections and tumor borders. Evidence of ascites and omental involvement were also assessed. Our study showed that benign ovarian serous tumors had a similar appearance to low grade malignant serous tumors, and were undistinguishable from the borderline serous carcinoma. The poorly differentiated serous adenocarcinoma was characterized by the presence of thick papillary projections rather than echogenicity. However, benign or malignant mucinous tumors gave the same pattern. Loss of tumor wall definition, ascites and omental involvement may signal malignancy. The dermoid tumor had a characteristic sonographic appearance.