AIM:To assess the diagnostic ability of endoscopic ultrasonography (EUS) for evaluating causes of distal biliary strictures shown on endoscopic retrograde cholangiopancreatography (ERCP) or magnetic resonance cholangiopancreatography (MRCP),even without identifiable mass on computed tomography (CT).METHODS:The diagnostic ability of EUS was retro-spectively analyzed and compared with that of routine cytology (RC) and tumor markers in 34 patients with distal biliary strictures detected by ERCP or MRCP at Dokkyo Medical School Hospital from December 2005 to December 2008,without any adjacent mass or ec-centric thickening of the bile duct on CT that could cause biliary strictures.Findings considered as benign strictures on EUS included preservation of the normal on EUS included preservation of the normal sonographic layers of the bile duct wall,irrespective of the presence of a mass lesion.Other strictures were considered malignant.Final diagnosis of underlying diseases was made by pathological examination in 18 cases after surgical removal of the samples,and by clinical follow-up for>10 mo in 16 cases.RESULTS:Seventeen patients (50%) were finally diagnosed with benign conditions,including 6normalsubjects,while 17 patients (50%) were diagnosed with malignant disease.In terms of diagnostic ability,EUS showed 94.1% sensitivity,82.3% specificity,84.2%positive predictive value,93.3% negative predictive value (NPV) and 88.2% accuracy for identifying malignant and benign strictures.EUS was more sensitive than RC (94.1% vs 62.5%,P=0.039).NPV was also better for EUS than for RC (93.3% vs 57.5%,P =0.035).In addition,EUS provided significantly higher sensitivity than tumor markers using 100 U/mL as the cutoff level of carbohydrate antigen 19-9 (94.1% vs53%,P=0.017).On EUS,biliary stricture that was fi-nally diagnosed as malignant showed as a hypoechoic,irregular mass,with obstruction of the biliary duct and invasion to surrounding tissues.CONCLUSION:E US can diagnose biliary strictures caused by malignant tumors that are undetectable on CT.Earlier detection by EUS would provide more therapeutic options for patients with early-stage pancreaticobiliary cancer.