The upper limit of the normal bile-duct diameter is significantly smaller by ultrasound than that generally accepted for radiographic techniques. This appears to be due to (a) radiographic magnification, (b) ultrasonic underestimation, (c) a possible choleretic effect of radiographic contrast material, and (d) the fact that different regions are measured with different techniques. The diameter of the common hepatic duct was measured by ultrasound in 30 patients prior to and during intravenous cholangiography, and these measurements were correlated with each other and with the radiographic measurement. In vitro studies were also performed. It was found that the choleretic effect significantly increased duct size in a small percentage of cases. The radiographic magnification was a factor of 1.3, and the ultrasonic diameter was about 1.5-2.0 mm too small. Perhaps the most important cause of the discrepancy was measurement of different regions with different techniques.