Purpose <p>To evaluate the diagnostic utility of bile duct attenuation measured on contrast-enhanced computed tomography (CT) in differentiating choledocholithiasis, malignant biliary obstruction, benign biliary obstruction, and cholangitis.</p> Methods <p>This retrospective study included 208 patients who underwent contrast-enhanced abdominal CT followed by endoscopic retrograde cholangiopancreatography (ERCP) within three days. Intraductal bile attenuation was measured using two region of interest (ROI) techniques (elliptical and 4-point), with all measurements obtained strictly from bile-filled ductal segments. Secondary morphologic features were also assessed. Diagnostic performance was evaluated using receiver operating characteristic (ROC) analysis, and multivariate logistic regression identified independent predictors of malignancy.</p> Results <p>Mean attenuation values were significantly higher in malignant obstruction and choledocholithiasis than in benign biliary obstruction and cholangitis (<i>p</i> &lt; 0.001). For malignant obstruction, the 4-point method yielded 96% sensitivity and 81% specificity at a cut-off of ≥ 29 HU (AUC: 0.97), while the elliptical method yielded 100% sensitivity and 80% specificity at ≥ 60 HU (AUC: 0.95). Elliptical ROI attenuation and bile duct wall thickness were independent predictors of malignancy (<i>p</i> = 0.010 and <i>p</i> &lt; 0.001, respectively). Interobserver agreement was excellent for both techniques (ICC &gt; 0.89).</p> Conclusion <p>When measured strictly within bile-filled ductal segments, CT-based attenuation provides useful quantitative information for differentiating common causes of biliary obstruction. Combined with ductal wall thickness and secondary morphologic features, attenuation analysis may enhance the diagnostic value of routine contrast-enhanced CT, particularly in settings where advanced imaging modalities are limited or delayed.</p> Graphical Abstract <p></p>