In order to calculate the percentage stenosis of an artery, the residual lumen and the normal dimensions at the site of stenosis must be known. Although the residual lumen can be measured directly from an arteriogram, the normal dimensions cannot in the presence of atherosclerotic narrowing. Most calculations of percentage stenosis are based on assumptions of what the true diameter of the vessel would have been before the onset of atheroma. Measurements of the diameter of the common, internal and external carotid arteries were made at standard sites in 61 normal arteriograms. There was a wide range of dimensions and a statistically significant difference between men and women. A linear relationship between the diameter of the proximal common carotid artery CCA(P) and the diameter of the bulb of the internal carotid was found (r = 0.74). There was a poor correlation between the lumen diameter of the distal internal carotid, which is most commonly used for determining the percentage stenosis, and the bulb (r = 0.41). Dividing each measurement by the diameter of the proximal common carotid to express the figures as a ratio produced a reduction of the range of measurements and abolished the difference between men and women. The mean ratio and standard deviation of the internal carotid bulb to the common carotid was 1.19 +/- 0.09, the distal internal to common carotid was 0.65 +/- 0.07 and the external to common carotid was 0.58 +/- 0.10.(ABSTRACT TRUNCATED AT 250 WORDS)