PURPOSE: To evaluate the accuracy of CT using water as an oral contrast material in a prone position in determining the depth of tumor invasion in gastric cancer patients. MATERIALS AND METHODS: Thirty three patients(19 male, 14 female) with surgically confirmed gastric cancer were studied. We performed CT in a prone position after ingestion of 1 liter of pure water. CT findings were classified into 4 groups by the morphologic appearances of infiltrates in the perigastric fat plane :normal perigastric fat(SO), fine mottled densities(S1), irregular aggregated or linear densities(S2) and direct extension and invasion of tumor into contiguous structures(S3). Also we prospectively compared the CT staging with pathologic T staging according to the TNM systems. RESULTS: The overall accuracy of CT staging in determining the pathologic T factor was 69.6%. As we regarded T1 and T2 lesions as one group on CT, the accuracy of CT staging was increased to 80.2% because of a limitation of CT for distinguishing T1 from T2 lesions. CONCLUSION: Prone position CT using water as an oral contrast agent is quite accurate in determining the T staging of gastric carcinoma.