Hypertrophic cardiomyopathy (HCM) is a disease of unknown etiology characterized by cardiac hypertrophy and disarrays of myocardial fiber and fibrils. More than half of patients with HCM show an apparent family history consistent with autosomal dominant inheritance. Mutations in the cardiac beta myosin heavy chain (MHC) gene have recently been identified in several Caucasian HCM families and suspected to be causative. To date, 8 missense mutations had been reported in Caucasian HCM families. We have also analyzed the structure of cardiac beta MHC gene in Japanese patients with HCM, and found 3 missense mutations. The application of the techniques of molecular biology provides a new understanding of HCM, for example, preclinical diagnosis and prediction of prognosis.