PURPOSE: To assess and compare the performance of CT and 18F-FDG-PET/CT in the evaluation of peritoneal carcinomatosis (PC). METHOD AND MATERIALS: Thirty consecutive patients with PC and scheduled for a surgery underwent a CT of the abdomen and pelvis and a whole-body 18F-FDG PET/CT. The extent of PC was assessed precisely using the peritoneal cancer index combining the distribution of tumor throughout 11 abdominopelvic regions with a lesion size score. CT and PET/CT imaging results were compared in all patients with intraoperative findings using an interclass correlation test. RESULTS: The presence of PC was correctly determined on CT and PET/CT in 23/28 and 16/28 patients, respectively. The extent of PC was understaged with CT and PET/CT in 27 patients and overstaged with CT and PET/CT in 1 and 2 patients, respectively. The interclass correlation was 0.53 (moderate) between CT and surgery and 0.12 (low) between PET/CT and surgery. The interclass correlation was higher for mucinous tumor (0.63) than for non-mucinous (0.16) on CT imaging whereas no difference was found in PET/CT. CONCLUSION: The intraperitoneal assessment of the extent of carcinomatosis, necessary to assess prognosis and treatment planning, is not accurate enough with CT and PET/CT imaging.