The deletion polymorphism of angiotensin converting enzyme (ACE) genotype has been reported as an independent risk factor for the development of myocardial infarction (MI). However there are conflicting data showing no relationship between the ACE genotype and coronary artery disease. The present study was performed to investigate the correlation between ACE genetic polymorphism and acute coronary syndrome by comparing the distribution of ACE genotypes and ACE activities in patients with acute MI and unstable angina with those in control group. The frequency of genotype DD was significantly higher in patients with acute coronary syndrome than in controls. Logistic regression analysis showed that ACE polymorphism affected the development of acute coronary syndrome in recessive pattern of D allele. When we divided the patients into MI and unstable angina groups, the frequencies of genotype DD and D allele were significantly higher in unstable angina group than in MI or control groups. In the patients with MI, the frequency of D allele was significantly higher in patients without previous angina than in those with previous angina. There was no significant difference in ACE genotype or allelic frequency according to the severity of coronary lesions. The ACE genotype was associated with marked differences of ACE activity, but there was no difference between the patient and control groups for each genotype. In conclusion, the genotype DD of ACE gene associated with acute coronary syndrome, but not with the severity of coronary artery lesion. These results showed that the genotype DD of ACE gene might be associated with acute coronary syndrome by another mechanism rather than the coronary atherosclerosis.