Abdominal CT yields several pieces of information for the diagnosis of secondary neoplastic involvement of the peritoneum (peritoneal carcinosis). Peritoneal carcinosis may be the first clinical sign of an occult primary tumor. CT scans of 120 patients with peritoneal carcinosis confirmed by pre/peri-operative biopsy or at autopsy, were retrospectively reviewed. The CT patterns of peritoneal carcinosis were detected in 20 of 44 patients with an occult malignancy. In 16 of 76 cases with a known malignancy there was no evidence of secondary peritoneal involvement. The incidence of CT findings and their correlation with the primary tumor are critically discussed. The authors have assessed the utility of CT in detecting peritoneal carcinosis, even though in their experience CT findings of secondary neoplastic peritoneal involvement could not be correlated with primary tumor.