Background: Cardiac rhabdomyomas associated with tuberous sclerosis induce various abnormalities in the electrocardiogram. Electrocardiographic evidence of ventricular hypertrophy may appear if the tumour is electrically active. To our knowledge, electrocardiographic evidence of ventricular hypertrophy has been reported only in association with congestive heart failure. Follow-up studies of changes in electrocardiographic findings are also lacking. Methods: We studied 21 consecutive patients with cardiac rhabdomyoma associated with tuberous sclerosis, 10 males and 11 females, aged from the date of birth to 9 years at diagnosis. The mean period of follow-up was 53 months. None of the patients developed congestive heart failure. We evaluated the electrocardiographic changes during the follow-up, and their association with echocardiographic findings. Results: Of the 21 patients, 12 showed one or more abnormalities on the electrocardiogram at presentation, with five demonstrating right or left ventricular hypertrophy. In all of these five cases, the tumours were mainly located in the respective ventricular cavity. In one patient with a giant tumour expanding exteriorly, there was marked left ventricular hypertrophy on the electrocardiogram. Followup studies showed spontaneous regression of the tumours in 12 of 19 patients, with abnormalities still present in only 7 patients. A gradual disappearance of left ventricular hypertrophy as seen on the electrocardiogram was noted in the patient with marked left ventricular hypertrophy at presentation in parallel with regression of the tumour. Conclusions: The presence of cardiac rhabdomyomas in patients with tuberous sclerosis might explain the ventricular hypertrophy seen on the electrocardiogram through its electrically active tissue without ventricular pressure overload or ventricular enlargement, although pre-excitation might affect the amplitude of the QRS complex. Even in cases with large tumours, nonetheless, the electric potential might not alter the surface electrocardiogram if the direction of growth of the tumour is towards the ventricular cavity. In many cases, electrocardiographic abnormalities tend to disappear, concomitant with regression of the tumours.