Objective:To evaluate the diagnostic value of relative lesion length in differentiating extrahepatic bile duct infiltrating cholangiocarcinoma with inflammation.Methods:From October 2014 to February 2018, 24 cases of infiltrating extrahepatic cholangiocarcinomas confirmed operatively and pathologically and 23 cases of extrahepatic bile duct inflammation confirmed clinically from the Third People′s Hospital of Datong City were respectively enrolled in this study. Upper abdomen MR and/or CT image data of all patients were respectively reviewed. The extrahepatic duct wall was defined as wall thickening with obvious enhancement. The length of the lesion was measured. L lesion/L duct was referred as the ratio of the lengths of lesion to extrahepatic bile duct (common hepatic duct+common bile duct)was calculated. The difference in the average values of L lesion/L ductbetween the cholangiocarcinomas group and inflammation group was analyzed with t test, and the differential diagnostic efficacy of L lesion/L ductratio was analyzed with receiver operating characteristic curve (ROC) test. Results:Significant difference was found in the length of lesion between the extrahepatic cholangiocarcinoma group [(22.01±1.86) mm] and the cholangitis group [(47.36±2.81) mm] ( P<0.01). The average ratio of L lesion/L ductwere 0.26±0.02 for the cholangiocarcinomas group and 0.54±0.03 for the inflammation group, respectively ( P<0.01). The area under the ROC curve of L lesion/L duct in diagnosis of the infiltrating extrahepatic cholangiocarcinomas was 0.92. With <0.40 as cut-off point, the diagnostic sensitivity and specificity were 87.5% and 82.6%, respectively. Conclusion:The L lesion/L ductmight be taken as an important diagnostic sign in differentiation between infiltrating extrahepatic cholangiocarcinomas and extrahepatic bile duct inflammation.