To retrospectively evaluate criteria for differentiating extrahepatic bile duct cholangiocarcinoma from benign cause of stricture at magnetic resonance cholangiopancreatography (MRCP) and to compare diagnostic accuracy with this modality versus endoscopic retrograde cholangiopancreatography (ERCP).MRCP and ERCP images in 50 patients (27 with cholangiocarcinoma [18 men, nine women; mean age, 58 years] and 23 with benign cause of stricture [13 men, 10 women; mean age, 60 years]) were retrospectively reviewed to assess the appearance of bile duct strictures. Final diagnosis was based on surgical or biopsy findings. Strictures were described according to their imaging appearance (irregular or smooth margins, asymmetric or symmetric narrowing, abrupt narrowing or gradual tapering, and presence or absence of double-duct sign). Sensitivity, specificity, and accuracy of MRCP and ERCP were calculated by using ratings of confidence in image-based diagnosis. Lengths of stricture were electronically measured and compared by using the Student t test.Among cholangiographic criteria for malignant biliary stricture, irregular margins and asymmetric narrowing were more common in cholangiocarcinomas (24 [89%] of 27 patients) than in benign strictures (six [26%] and eight [35%] of 23 patients, respectively). Sensitivity, specificity, and accuracy of the two methods for differentiation of malignant from benign causes of biliary stricture were 81% (22 of 27), 70% (16 of 23), and 76% (38 of 50), respectively, for MRCP and 74% (20 of 27), 70% (16 of 23), and 72% (36 of 50), respectively, for ERCP. Mean length (+/- standard deviation) of cholangiocarcinomas was 30.0 mm +/- 8.5, and that of benign strictures was 13.6 mm +/- 9.1 (P <.001).Accuracy of MRCP is comparable with that of ERCP. Regardless of modality, a lengthy segment of extrahepatic bile duct stricture with irregular margin and asymmetric narrowing suggests cholangiocarcinoma, and a short segment with regular margin and symmetric narrowing suggests benign cause.