The upper limit of the normal extrahepatic duct diameter when measured by sonography in our institution is less than half that when measured by endoscopic retrograde cholangiopancreatography (ERCP). The objective of this study was to locate possible sources of this discrepancy by comparing measurements obtained on the same patient by sonography, before and after ERCP. Thirty consecutive patients referred for ERCP were entered into a prospective trial; bile duct measurements were obtained independently by both techniques in 19 patients. Correcting for radiographic magnification, the ERCP measurement was more than twice that obtained by ultrasonography. Among the possible reasons for diverse results that we considered, radiographic magnification, ultrasonic underestimation, and distending effects of retrograde cholangiography (ERCP) were not found to be prominent causes for the marked discrepancy that we observed. The most likely explanation is that the duct in individual patients was being measured at a different level by the two techniques. A retrospective comparison of the studies obtained by each method suggests that the sonographic measurement is most often of the right hepatic duct. If this is the case, the two measurements show no statistically significant difference (P > 0.05). A prospective trial is needed to test this hypothesis.