PURPOSE: To determine the incidence of flow artifact and vascular compression, phenomena that mimic biliary stone disease at magnetic resonance cholangio pancreatography (MRCP). MATERIALS AND METHODS: In 160 patients who underwent MRCP, the prescence and location of flow artifact were determined. The signal intensity of flow artifacts was chassifieded as either higher than renal cortical density (group I), the same as renal cortical density (group II), the same as hepatic density (group III), or the same as vascular density (group IV). Correlation between flow artifact and the largest diameter of the extrahepatic duct (EHD) was statistically evaluated, and the location of vascular compression in the biliary system and causative vessels was also determined. RESULTS: At MRCP, flow artifacts were observed in 81 patients (76.4%). Forty-five (42.5%) were classified as group I, 15 (14.2%) as group II, 18 (17.0%) as group III, and three (2.8%) as group IV. They were located in the common bile duct (78.3%), common hepatic duct (70.0%), or intrahepatic duct (29.2%) or at the cystic duct insertion site (7.5%). In patients in whom a flow artifact was not apparent, the diameter of the EHD was 7.1mm; in those with an artifact, this diameter was 11.3 mm. The mean diameter of the EHD was greater in groups II, III and IV (11.4 mm) than in group I (9.8 mm). Vascular compression was demonstrated in 21 patients (19.8%), occurring in the common hepatic duct in 8.5%, the left intrahepatic duct in 8.5%, the common bile duct in 1.9%, and the right intrahepatic duct in 0.9%. Causative vessels were the right hepatic artery (12.5%), left hepatic artery (5.7%), and branches of the gastroduodenal artery (1.9%). CONCLUSION: As the extrahepatic duct is wide, a flow artifact appears and signal intensity decreases. In particular, flow artifacts with a signal intensity of grade III or IV, occuring in 19.8% of patients, mimicked biliary stones at MRCP. The presence of a flow artifact and vascular compression, which mimic biliary stone, therefore be carefully interpreted.