In this study we have performed human leukocyte antigen (HLA)-A and B typing on nine patients with hypertrophic cardiomyopathy (HCM) and their relatives. Four patients had relatives who also had the disease. HLA typing of the familial form of HCM revealed a very close association of a given HLA-A,B haplotype with the occurrence of the disease. All four patients who had affected relatives had obstruction of left ventricular outflow (LVOT), while four patients with unaffected relatives did not have obstruction. One additional patient with obstruction and without familial incidence was an only child and had few living relatives. Thus, HCM can be divided into two subtypes: a familial form linked to the HLA-A,B system, which may be related to obstructive type, and a sporadic form not linked to HLA antigens. These data confirm the existence of at least two separate forms of hypertrophic cardiomyopathy. The study also confirms their existence in the Japanese population, with a completely different gene pool than the population from the southeastern United States in whom the observation was initially described.