Using a high-resolution real-time scanner, a technique has been established in our laboratory for visualizing the entire extrahepatic bile duct with multiple longitudinal oblique approaches and, if necessary, several transverse scans through the pancreatic head. To determine the normal size of the extrahepatic bile duct and to evaluate the effect of aging on the duct diameter, 256 healthy subjects and patients without hepatobiliary disease were examined using this technique from January to August, 1982. The entire extrahepatic duct was detected in 203. The inner diameter of the widest point of the duct varied from 1 mm to 10 mm, and was found to be age-dependent (r = 0.60, P less than 0.001). It is concluded that, based on the visualization of the entire extrahepatic duct echographically, the normal inner diameter of common bile duct can be up to 10 mm, and there is a mild change of duct size with age.