We determined the prevalence of mutations considered malignant in the genes for beta-myosin heavy chain (MYH7, 11 mutations) and troponin T (TNNT2, 5 mutations) in 30 patients with hypertrophic cardiomyopathy aged 18 to 60 years, 83% of whom had familial antecedents of hypertrophic myocardiopathy or sudden death. Mutations were identified with polymerase chain reaction followed by restriction enzyme digestion and agarose gel electrophoresis. Direct analysis identified 16 mutations in 2 of the 30 patients (7%): one women diagnosed at the age of 25 years as carrying the MYH7453cysteine mutation, and a 60-year-old women with the TNNT2278 cysteine mutation. These cases illustrate the considerable clinical heterogeneity that characterizes carriers of these mutations. Clinical manifestations can range from severe hypertrophy or early sudden death to the absence of symptoms up to advanced age.