A deletion/insertion polymorphism in the ACE gene has been reported previously as a potent factor for myocardial infarction. We have tested the frequency of the deletion (D) allele of the ACE gene in 308 consecutive patients admitted to coronary care with chest pain. The gene frequencies were compared with those of 348 controls recruited from the London area. Of 108 Caucasian patients with myocardial infarction, the DD genotype was found more frequently than the combined DI and II genotypes (Chi-square, chi 2 = 5.07, 2P = 0.024). The overall D gene frequency was higher in myocardial infarction patients (125 of 216, 58%) than in controls (347 of 696, 49.9%) (chi 2 = 3.79, 2P = 0.052). In contrast, the DD genotype and D allele frequencies in patients with unstable angina were similar to those found in our normal population. A nonsignificant difference in allele frequency between myocardial infarction and unstable angina patients was observed but the small numbers of subjects studied precludes a more formal comparison. Since unstable angina and myocardial infarction represent a spectrum of coronary thrombosis, it is possible that the DD genotype favours the development of myocardial infarction, perhaps through the presence of higher serum ACE concentrations.