30 children with hypertrophic cardiomyopathy and a mean age of 4.9 +/- 4.4 years, 11 infants and 19 children, were followed up for an average of 6.6 +/- 5.6 years. Apart from a systolic murmur in nearly all patients, only a few had symptoms. 7 patients from 5 families had a positive family history. The ECG often showed left ventricular hypertrophy, twice a deep Q wave, and no cases of giant T wave. A subaortic pressure gradient greater than or equal to 40 mm Hg was measured in 6 children, a secondary cardiomyopathy was excluded by biopsy in 8. Associated lesions were coarctation (n = 2) and patent ductus arteriosus (n = 2). 7 children died: 1 infant in congestive heart failure, 3 children suddenly, and 3 postoperatively. The annual mortality rate was 3.5%. Also in childhood, beta-blocker therapy may not prevent sudden cardiac death, nor may operation prevent late complications. In spite of the lack of pressure gradients, the children did not fulfil the criteria of hypertrophic non-obstructive cardiomyopathy.