Echocardiography was used to detect the familial prevalence of asymmetric septal hypertrophy in relatives of patients with proven idiopathic hypertrophic subaortic stenosis. Idiopathic hypertrophic subaortic stenosis is only one clinical expression of a cardiomyopathic disease spectrum, including asymptomatic patients having asymmetric septal hypertrophy as a characteristic anatomic marker which can be detected by echo. The validity of previous proposed criteria to detect this marker was checked in our population. Therefore we examined normal subjects, patients with fixed left ventricular outflow obstruction (valvular aortic stenosis), and those with idiopathic hypertrophic subaortic stenosis who served as index cases. A septal thickness exceeding that of the free left ventricular posterior wall by 30% separates patients with a cardiomyopathy from those without this disease. 27 of 73 examined relatives of 14 index cases were found to have asymmetric septal hypertrophy. In those instances where information was available from the parents of the index cases, one parent was found to be affected. When the examined group is considered from a parent-child relationship (including the index case when appropriate), it included 78 children of affected single parents of which 20 males and 19 females had asymmetric septal hypertrophy. The history, clinical examination and electrocardiogram were not useful to detect the disease. The results suggest an autosomal dominant mode of inheritance of asymmetric septal hypertrophy with a high penetrance.