The diagnosis of peritoneal mesothelioma was made prospectively and noninvasively in four patients with the use of sonography, computed tomography, and sonographically guided fine-needle aspiration biopsy. The imaging methods revealed information similar to the operative findings, with clear superiority of computed tomography over sonography. These noninvasive methods may be used as screening tools, especially among groups or in regional areas with a high risk for asbestos exposure. The findings included soft-tissue masses with invariable involvement of the omentum; small intraperitoneal nodules; thickened peritoneum, mesentery, and bowel wall; pleural plaques; and usually minimal, if any, ascites. Since the differential diagnosis from peritoneal carcinomatosis may be difficult, sonographically (or CT) guided aspiration biopsy is needed to produce diagnostic cytologic specimens. The use of this type of biopsy should obviate surgical exploration.