PURPOSE: To determine whether an analysis of two-phase spiral CT features provides a sound basis for differential diagnosis between gallbladder carcinoma and cholecystitis. MATERIALS AND METHODS: We reviewed a total of 89 cases of gallbladder carcinoma (n=35) or cholecystitis (n=54) in patients who had undergone two-phase spiral CT. For this, a GE Highspeed Advantage scanner (GE Medical Systems, Milwaukee, U.S.A.) was used. A total of 120ml of contrast material was injected at a rate of 2 -3 ml/sec. Arterial and venous phase scans were obtained 35 and 65 seconds, respectively, after the initiation of contrast infusion. All cases of gallbladder carcinoma and 468 of cholecystitis (of a total of 482) were confirmed by histopathology. We reviewed the two phase spiral CT features, analyzing and assessing thickness of the lesion, the enhancement pattern seen during the arterial and the venous phase, invasion of the liver, pericholecystic fat infiltration, dilatation of intrahepatic ducts, and other associated findings. RESULTS: Mean wall thickness was 12.6 mm in the gallbladder carcinoma group, and 7.2 mm in the cholecystitis group. The common enhancement patterns seen in gallbladder carcinoma were a highly enhanced thick inner wall layer during the arterial phase which became iso attenuated with adjacent liver parenchyma during the venous phase (16/35; 45.7%), and 2) a highly enhanced thick inner wall layer during both the arterial and the venous phase (8/35; 22.9%). The most common enhancement pattern in cholecystitis cases was an iso attenuated thin inner wall layer during both the arterial and the venous phase (44/54; 81.5%). Findings of intrahepatic mass formation by direct invasion (9/35), lymph node enlargement (12/35), and metastasis to other organs (7/35) occurred only in cases of gallbladder carcinoma. Dilatation of intrahepatic ducts was more frequent in cases of gallbladder carcinoma (18/35, 51.4%) than of cholecystitis (10/54, 18.5%). The incidence of pericholecystic fat infiltration and fluid collection was not significantly different between the gallbladder cancer and cholecystitis groups. CONCLUSION: Gallbladder carcinoma and cholecystitis varied in terms of wall thickness, enhancement pattern, and intrahepatic ductal dilatation, as seen on two phase spiral CT. Findings of liver invasion, lymph node enlargement and distant metastasis strongly suggested gallbladder carcinoma. These results suggested that gallbladder carcinoma and cholecystitis can be distinguished by analysis of their two phase spiral CT features.