The purpose of this study was to provide basic criteria for interpreting images of the normal duodenal papillary region obtained by intraductal ultrasonography at the frequency of 20 MHz. Our in vitro examination of autopsy specimens from 15 patients revealed that the images could be classified into three patterns according to the spatial relationships between the duodenal muscularis propria and the bile duct, or the common duct. Oddi's muscle was clearly demonstrated surrounding the mucosa of the bile duct or the common duct, which was visualized as a hypoechoic layer. The images obtained using in vivo examination of 60 patients with pancreato-biliary disease via either the percutaneous or the peroral approach were similar to the images obtained in vitro. In eight patients with cancer of the papilla of Vater, the tumor was demonstrated clearly on intraductal sonograms. The intraductal imaging features of the normal papillary region were clarified, and the clinical usefulness of this technique in the evaluation of the tumor extent in patients with cancer of the papilla of Vater is suggested.