Stenting of the internal carotid artery is facilitated by stenting across the carotid bifurcation and sizing the diameter of a self-expanding stent to the large common carotid segment. This usually results in marked oversizing of the self-expanding stent in the internal carotid segment. This study was done to determine the relationship between stent oversizing and late luminal loss index after stenting of the internal carotid artery. Between September 1995 and March 1997, there were 165 patients (189 vessels) who underwent successful carotid stenting with self-expanding stents. Fifty-nine patients (63 vessels) had six-month follow-up carotid angiograms and on-line quantitative angiographic analysis. The mean reference diameter of the internal carotid arteries was 4.93+/-1.31 mm. Nominal stent size was 5 mm in 4 patients, 6 mm in 6 patients, 8 mm in 106 patients, 10 mm in 77 patients, and 12 mm in 1 patient. The average stent/patient was 1.03+/-0.16. There were three patients who had more than 50% diameter renarrowing at follow-up. The mean late loss index was 0.25+/-0.41. By linear regression analysis, there was no clear linear relationship between stent oversizing and late loss index after stenting (correlation coefficient = -0.21, P = 0.09). When analysis of variance with linear contrast was used to analyze six groups of different stent/artery ratios (from 1.4 to > or = 2), late loss indexes are significantly lower in the groups of high stent/artery ratio than the groups of low stent/artery ratio (P = 0.01). The process of oversizing of self-expanding stents deployed in the internal carotid artery does not appear to be associated with late restenosis and high stent/artery ratio seems to be associated with low late loss index.