Retrospective analysis was performed to assess the natural history in relation to clinical and haemodynamic features in 37 patients in whom hypertrophic cardiomyopathy had been diagnosed in childhood. At diagnosis they were aged 1 to 14 years (mean 9 years). Eighteen presented with chest pain and either dyspnoea or symptoms of impaired consciousness or both; 19 were asymptomatic and were referred for evaluation of abnormalities detected during physical examination. During a mean follow up of nine years, 18 patients died, a cumulative annual medical mortality of 4.8%. Five patients experienced severe dyspnoea or chest pain: two of these had progressive dyspnoea and died in cardiac failure, and three died after myectomy. The 19 survivors were compared with the 11 sudden deaths. Eleven of the survivors and five of the sudden deaths were asymptomatic. Of 18 clinical, electrocardiographic, and haemodynamic features only syncope and electrocardiographic evidence of right ventricular hypertrophy were associated with sudden death. In children with hypertrophic cardiomyopathy sudden death was common and was not well predicted by clinical, electrocardiographic, or haemodynamic findings. Hypertrophic cardiomyopathy indicates a poor prognosis even if symptoms are absent or mild.