The degree of magnification in radiographic imaging of the bile duct has not been sufficiently emphasized. Differences in magnification account for much of the considerable discrepancy among various radiographic and nonradiographic measurements of size. In measuring a series of 22 freshly extracted common bile duct stones, an average magnification factor of 1.43 (43%) was found on fluoroscopic spot films; this compares with the 1.3 (30%) magnification generally accepted as being present in conventional intravenous cholangiography using the overhead-tube technique. The large magnification factor must be remembered in interpreting fluoroscopic images of the bile duct. Most published dimensions are meaningless unless accompanied by a statement of technique and magnification factor.