The definitive diagnosis of peritoneal mesothelioma and its differentiation from metastatic peritoneal carcinomatosis may be difficult because of the clinical, macroscopic, and microscopic variability of the tumor. To this purpose, a combination of criteria, including the clinical picture, the gross pathologic findings, the exclusion of other primary neoplasms, and the microscopic findings, must be taken into consideration. Conventionally, these criteria may be established only after surgical exploration and extensive sampling. Our experience with patients with peritoneal mesothelioma and metastatic peritoneal carcinomatosis, as well as a review of the recent imaging literature, shows excellent correlation between computed tomography or ultrasound and the operative or autopsy findings. These imaging modalities showed soft-tissue masses or nodules; thickened omentum ("omental cake"), peritoneum, mesentery, and bowel wall; pleural plaques; and usually disproportionally small, if any, ascites. The latter two observations may be useful in differentiating mesothelioma from carcinomatosis macroscopically. Furthermore, fine-needle aspiration biopsy, after performing wide sampling of the tumors in different locations under ultrasonic or computed tomographic guidance, produced diagnostic cytologic specimens. Thus, the need for exploratory surgery may be alleviated, and the diagnosis of peritoneal mesothelioma may be made prospectively and relatively noninvasively with the use of computed tomography or ultrasound and fine-needle aspiration biopsy. Since epidemiologic studies predict increasing incidence of this neoplasm, especially among asbestos workers, it is suggested that these techniques be seriously considered as screening methods for high-risk populations.