Abstract To investigate the relation between the angiotensin-converting enzyme (ACE) gene polymorphism and acute coronary syndromes with respect to environmental factors, we analyzed the association of genotype with the coronary angiographic findings of patients with acute myocardial infarction or unstable angina pectoris, and we examined the linkage of each genotype with established risk factors for coronary artery disease. We determined the ACE genotype in 152 Japanese patients with acute coronary syndromes and 399 healthy individuals. The genotype distributions were not different between the two groups ( P =.74, χ 2 test). In the former group, coronary angiograms were evaluated by criteria based on the number of diseased vessels, the number of stenotic lesions (≥50%), and the relative abnormal arterial portion (extent index). Although the number of stenotic lesions was higher in patients with the DD genotype than in those with the ID or II genotype ( P =.006), there were no differences in the number of diseased vessels or the extent index. When only smokers were analyzed, the number of diseased vessels ( P =.032), number of stenotic lesions ( P =.003), and extent index ( P =.019) were all higher in patients with the DD genotype than in those with the ID or II genotype. In contrast, these differences in the respective parameters did not exist in nonsmokers. The results indicate smoking-associated effects of the ACE genotype on the severity of coronary atherosclerosis.