The authors present the findings of seven hundred and fifty adult patients without biliary tract disease for whom the internal diameter of the common bile duct was measured. Six hundred and seventy-three patients had no biliary tract disease whatever while seventy-seven had no clinical evidence of disease but had undergone cholecystectomy over one year ago. In our series, the mean diameter of the common bile duct in normal patients was 3.67 +/- 0.03 mm. Only 5.9% of normal patients were found to have a bile duct with a diameter greater than or equal to 5 mm. The frequency of this finding increased in patients over fifty years of age. The mean diameter of the common bile duct in the seventy-seven who had undergone cholecystectomy was 4.47 +/- 0.10 mm.