Eleven patients with malignant peritoneal mesothelioma were examined with ultrasonography. Four of them were also examined with computed tomography (CT). A sheetlike mass was seen in seven patients and a localized irregular mass was seen in four. No nodular or globular masses were seen. Ascites was present in all patients except one, and an omental mass in addition to a peritoneal mass was found in two. Extremely rapid progression of mesothelioma occurred in one patient, and disappearance of mesothelioma and ascites occurred in another. Ultrasonography was, in general, superior to CT in demonstrating peritoneal masses, especially in the presence of ascites.