Retrospective analysis of the clinical course of 254 patients with hypertrophic cardiomyopathy, followed up for 1 to 23 years (mean 6), disclosed that 58 had died, 32 of them suddenly. The 196 survivors were compared with the 32 patients who died suddenly and with the 38 who died suddenly or with heart failure. The combination of young age (14 years or less), syncope at diagnosis, severe dyspnea at last follow-up and a family history of hypertrophic cardiomyopathy and sudden death best predicted sudden death (false negative rate 30 percent, false positive rate 27 percent). A "malignant" family history was associated with poor prognosis, particularly in the younger patients; a family history of hypertrophic cardiomyopathy without sudden death was more frequent in the survivors (12 percent) than in the dead (5 percent). Patients who had a diagnosis in childhood were usually asymptomatic, had an unfavorable family history and a 5.9 percent annual mortality rate. In those aged 15 to 45 years at diagnosis, there was a 2.5 percent annual mortality rate and syncope was the only prognostic feature. Among those diagnosed between age 45 and 60 years, dyspnea and exertional chest pain were more common in the patients who died, and the annual mortality rate was 2.6 percent. Poor prognosis was better predicted by the history at the time of diagnosis and by changes in symptoms during follow-up than by an electrocardiographic or hemodynamic measurement.