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Introduction The angiotensin-converting enzyme (ACE) DD-genotype is associated with increased plasma and myocardial ACE-activity. The influence of the ACE insertion/deletion (I/D) polymorphism on the effects of ACE-inhibition on vascular responses has not been previously described. Materials and methods In the randomised, double-blind QUinapril On Vascular ACE and Determinants of Ischemia Study (QUO VADIS), 149 patients undergoing coronary bypass surgery were randomised to receive either the ACE inhibitor, quinapril, or placebo. In 82 patients, we obtained ACE-genotype, and measured vascular responses to angiotensin II (Ang II) in left internal mammary arteries. Results In the placebo group, the mean maximal vasoconstriction to Ang II was significantly lower in patients with the DD-genotype than in those with the ID/II genotype (36.2±5.11% [n=13] vs. 55.6±4.57% [n=25]; p=0.01). In the quinapril group, the mean maximal vasoconstriction to Ang II was similar [n=8] vs. 57.7±4.07% [n=35]; p=0.85). between DD- and ID/II-genotype (59.6±9.19% Conclusions DD-genotype patients showed decreased vascular responses to Ang II but treatment with quinapril completely restored the decreased vascular response in DD-genotype patients to the same level as II/ID-genotype patients, while no effect of quinapril was demonstrated in the II/ID-genotype patients.