Computed tomography (CT) correctly diagnosed common bile duct stones in 19 (90%) of 21 surgically proven cases. CT demonstrated calculi in all 13 patients with calcium bilirubinate common duct stones and six of eight patients with predominantly cholesterol stones. There were two false-negative diagnoses in patients with cholesterol calculi scanned at 1 cm intervals. Cholesterol stones are nearly isodense with bile and may be difficult to visualize in the common duct by routine scanning techniques. There are three suggestive CT features of cholesterol common duct calculi: (1) abrupt termination of the distal common bile duct without an obstructing mass, (2) a faint rim of increased density along the peripheral margin of a low-density calculus, and (3) mottled areas of increased density centrally within a calculus elevating its CT number above values for normal bile. Modification of scanning technique to obtain high-resolution images of the extrahepatic biliary tree is stressed to optimize visualization of the above findings.