We studied 52 children in order to assess the prevalence, natural history, and electrophysiologic effects of cardiac rhabdomyoma in tuberous sclerosis. Their ages ranged from one day to 12 years (median age 1.33 year). The tumors in the heart were found in 22 patients through cross-sectional echocardiography. The overall prevalence of cardiac rhabdomyoma was 42%. The prevalence of cardiac tumors in patients first seen in infancy was 77%, and that in patients over one year of age was 31%. Those with cardiac tumors diagnosed in infancy had more tumors per person than did the patients diagnosed after infancy, and the tumors were smaller in the patients diagnosed over one year of age. Follow-up cross-sectional echocardiography showed that almost all tumors showed spontaneous regression. Complete regression occurred more commonly in younger patients, and in those with smaller tumors. About 70% of patients with cardiac tumors had either normal findings or infrequent premature contractions on 24hour Holter monitorings. The rest of the patients with cardiac tumors showed either frequent premature contractions, atrioventricular block, or atrial tachycardia, but antiarrhythmic medication was rarely required. Cross-sectional echocardiography, therefore, seems to be the best diagnostic tool for screening cardiac rhabdomyomas in patients with tuberous sclerosis. The tumors may be managed more conservatively in view of their high rate of spontaneous regression and rather rare association with symptomatic arrhythmias.