OBJECTIVE--To assess the incidence, importance, and history of cardiac involvement in infants and children with tuberous sclerosis. DESIGN--Prospective study; clinical examination, sector and Doppler echocardiography, standard and ambulatory electrocardiography. SETTING--A tertiary referral centre. PATIENTS--21 patients with tuberous sclerosis aged 1 day to 16 years (mean 6.3 years); follow up investigations were available in 14 cases (10 retrospective, 4 prospective; mean follow up 4.3 years). RESULTS--Multiple cardiac rhabdomyomas in the right ventricle (11) and left ventricle (14) as well as in the right atrium (1) were present in 14/21 patients. Two of them had obstruction of the left ventricular inflow and outflow tract related to a tumour. In the remaining 7 patients, echocardiography was normal in 4 and equivocal in 3 cases. The standard electrocardiogram (n = 20) showed ventricular hypertrophy (2), ventricular pre-excitation (1), arrhythmias (2), and repolarisation disturbances (4) in 7/13 patients with rhabdomyomas but was normal in all patients with a normal or equivocal echocardiogram. The ambulatory electrocardiogram (n = 19) showed frequent premature atrial (2) and polymorphous ventricular (2) contractions. The polymorphous ventricular contractions coexisted with rhabdomyomas. No arrhythmias that needed medical treatment were found. Follow up investigations showed return to a normal standard electrocardiogram in 3 patients. Definite regression or complete disappearance of the tumour occurred in 6 infants. CONCLUSIONS--Cardiac rhabdomyomas, although often present in these patients with tuberous sclerosis, caused neither major arrhythmias nor haemodynamic obstruction except in the neonatal period. The indication for operation is limited to cases with life threatening obstruction or arrhythmias refractory to medical treatment.