Restenosis after coronary artery stent implantation is attributed chiefly to intimal hyperplasia, which is prevented experimentally by angiotensin-converting enzyme (ACE) inhibitors. Therefore, the present study investigated whether the effect of quinapril, a tissue-specific ACE inhibitor, on the prevention of coronary restenosis differs according to ACE polymorphism. One hundred consecutive patients with successful stent implantation were randomly assigned to quinapril and control groups. Both follow-up angiography and ACE polymorphism analysis were obtained from 92 patients (control, 46; quinapril treatment, 46). The prevalence of risk factors did not differ statistically according to quinapril treatment or ACE genotypes. There was no statistically significant difference in the occurrence of restenosis 6 months after stenting between the groups. Quantitative coronary angiography revealed that quinapril treatment resulted in significantly higher minimal lumen diameter and significantly lower percent diameter stenosis (22.9±22.6 vs 37.1±19.7% in the control group, p<0.05) in patients with the D allele although there was no difference in those with the II genotype. In addition, intravascular ultrasound revealed that quinapril treatment significantly prevented the loss of minimal lumen cross-sectional area and the increase in percent area stenosis (34.5±14.0 vs 53.3±16.4% in the control group, p<0.05) in patients with the D allele compared to those with the II genotype. These results suggest that the administration of ACE inhibitors for the attenuation of lumen loss after coronary stent implantation is best for subjects with the D allele of the ACE genotype. (Circ J 2002; 66: 311 - 316)