We used an abnormally thick interventricular septum (greater than or equal to 1.3 cm) as an echocardiographic marker to find the inheritance pattern of hypertrophic cardiomyopathy among relatives of eight patients who had that disease at necropsy. Forty normal subjects served as a control group. Fifty-eight family members were examined and 18 (41%) of the 44 first degree relatives had hypertrophic cardiomyopathy. The overall inheritance pattern was consistent with an autosomal dominant genetic disorder and in one family a recessive trait could be excluded. The diagnosis of hypertrophic cardiomyopathy can be difficult clinically as only 13% of our patients had serious symptoms and only 30% had abnormal auscultatory findings. The electrocardiogram is a useful screening test among relatives as it was abnormal in 20 (87%) of those who had an abnormally thick septum. Symmetric septal hypertrophy was found in 30% of patients with cardiomyopathy in this study and only 17% had clinical evidence of obstruction.