Masses in the mesenteries and omentum are often difficult to diagnose by conventional radiographic techniques. Gray scale sonography was a valuable adjunct to radiographic vector analysis in four children with such masses. Masses that are clearly separable from the liver and spleen and do not distort identifiable extraperitoneal structures are probably intraperitoneal. In children most cystic intraperitoneal masses are related to the mesenteries, omentum, ovary, or bile ducts. An anterior fluid collection with internal septa (which might be mistaken for loculated ascites) is the typical sonographic appearance of an omental cyst. Echogenic masses are more difficult to evaluate: careful study of the acoustical features yielded important information in cases of omental lipoma and rhabdomyosarcoma metastatic to the mesenteries and omentum.