Purpose: To evaluate the added value of diffusion-weighted magnetic resonance (MR) imaging in diagnostic performance of conventional MR imaging for diagnosis of ampullary carcinoma. Materials and This retrospective study was institutional review board ap-Methods: proved, and informed consent was waived. Twenty-three patients with malignant ampullary obstruction and 39 patients with benign ampullary obstruction were included. Qualitative (signal intensity and enhancement pattern) and quantitative (apparent diffusion coefficient [ADC]) analyses were conducted for visible or expected locations of duodenal papillae. Two observers independently reviewed conventional MR images and subsequently reviewed combined conventional and diffusion-weighted MR images. A five-point scale for likelihood of ampullary carcinoma was used. Fisher exact test and Mann-Whitney U test were used for comparing groups, and diagnostic performance (receiver operating characteristic [ROC] curve analysis), accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were evaluated. Results: Visible or expected duodenal papillae in benign group showed isointensity (27-38 of 39, 69%-97%) and similar enhancement pattern (27 of 39, 69%) to that of normal duodenal wall more frequently than in malignant group (seven to 18 of 23 [30%-78%] and three of 23 [13%], respectively) on conventional MR images (P,.05). On diffusion-weighted images, 21 of 23 (91%) ampullary carcinomas showed hyperintensity, whereas all benign cases showed isointensity compared with normal duodenal wall (P,.001). Mean ADC of ampullary carcinomas (1.23 3 10-3 mm2/sec) was significantly lower than that of benign group (1.69 3 10-3 mm 2/sec) (P,.001). Diagnostic performance (area under ROC curve [Az]) of both observers improved significantly after additional review of diffusionweighted images; Az improved from 0.923 to 0.992 (P =.029) for observer 1 and from 0.910 to 0.992 (P =.025) for observer 2. In addition, diagnostic accuracy, sensitivity, specificity, PPV, and NPV of combined conventional and diffusion-weighted MR images were higher than those of conventional MR images alone. Conclusion: Addition of diffusion-weighted imaging to conventional MR imaging improves detection of ampullary carcinoma when compared with conventional MR imaging alone.