Objective:To assess the diagnostic accuracy of whole-body MRI using diffusion-weighted sequence (WB-DWI) to determine the peritoneal cancer index (PCI) in correlation with surgical and histopathological findings.Methods:Twenty-seven patients underwent preoperative WB-MRI, followed by cytoreductive surgery for primary tumors of the appendix (n=15), colorectum (n=12), and associated peritoneal disease.A total of 351 regions were retrospectively reviewed.The sensitivity, specificity, and accuracy were calculated at 13 anatomical sites.The WB-DWI PCI and PCI type were compared with surgical and histopathological findings.Results:No statistical difference was found between the WB-DWI PCI and surgical PCI (P=0.574).WB-DWI correctly predicted the PCI type in 24 of 27 patients with high accuracy (88.9％), including 10 of 10 patients with small-volume tumor, 12 of 14 with moderate-volume tumor, and 2 of 3 with large-volume tumor.WB-DWI correctly depicted tumors in 163 of 203 regions, with 40false-negative and 23 false-positive regions.The overall sensitivity, specificity, and accuracy of WB-DWI for the detection of peritoneal tumors were 80.3％, 84.5％, and 82.1％, respectively.For lesions＜0.5 cm in diameter, WB-DWI demonstrated good sensitivity (69.4％).Conclusions:WB-DWI accurately predicted PCI before surgery in patients undergoing evaluation for cytoreductive surgery.