The common hepatic and proximal common bile ducts (common duct) lie anteriorly and generally to the right of the portal vein in the porta hepatis. This constant anatomic relationship can be used to demonstrate the common duct and to differentiate it from the portal vein by gray scale ultrasonography. The patient is scanned longitudinally from the right anterior abdominal wall with the ultrasound beam directed posteromedially until two tubular structures are demonstrated in the porta hepatis. The more anterior tubular structure in this projection is the common duct. If the common duct is less than 3 mm in diameter, it may not be clearly delineated. In these cases the absence of common duct dilatation can be inferred. The usefulness of this view for detecting common duct dilatation was evaluated in a series of 101 consecutive cases proven by surgery, autopsy, or cholangiography. The overall accuracy was 96% (four false negative studies; no false positives). We conclude that sonography should be the imaging procedure of choice for suspected extrahepatic biliary obstruction if the serum bilirubin level precludes intravenous cholangiography.