To determine the mode of inheritance of hypertrophic cardiomyopathy 193 first degree relatives (parents, siblings, and offspring) of 50 patients with hypertrophic cardiomyopathy were assessed by clinical examination, electrocardiography, M mode and cross sectional echocardiography, and necropsy when available. Thirty nine (20%) first degree relatives had hypertrophic cardiomyopathy--37% of parents, 25% of siblings, and 8% of offspring. Eight (23%) of 35 affected relatives diagnosed by echocardiography had normal clinical and electrocardiographic findings. In the total study group 43% of the male population and 30% of the female population were affected. This difference is statistically significant. In 28/50 families there was familial occurrence of hypertrophic cardiomyopathy. Familial occurrence was demonstrated in 17 of 18 families in which five or more family members were assessed. In 15 families the pattern of inheritance was consistent with an autosomal dominant trait; in the other 13 the affected members were identified in a single generation and the pattern of inheritance could not be determined.