We determined the distribution frequency of angiotensin converting enzyme (ACE) polymorphism in 100 patients with hypertrophic cardiomyopathy and 106 of their unaffected siblings and offspring. The distribution of ACE genotypes was different in the two groups: allele D frequency of 0.69 in patients and 0.57 in relatives (p = 0.021). The frequency of allele D was also higher in hypertrophic cardiomyopathy families with a high incidence of sudden cardiac death than those with a low incidence (0.74 vs 0.55, p = 0.011). In 25 hypertrophic cardiomyopathy patients with a strong family history of sudden cardiac death the frequency of allele D was 0.82 (p = 0.001 vs normals).