Twenty-one patients, aged from 1 day to 16 years (median 8 months), with primary cardiac tumours, were studied with regard to the occurrence of arrhythmias. Standard electrocardiogram (ECG) was available in 20 cases and 24 h ambulatory ECG in 19; nine patients had prospective follow-up investigations (mean follow-up time 3.0 years). Standard ECG revealed preexcitation in two and arrhythmias in six patients. Ambulatory ECG disclosed seven patients with significant rhythm disturbances and in four of these, the arrhythmia was the symptom leading to the diagnosis of a cardiac tumour. Three patients were symptomatic from arrhythmia and two of them were successfully treated with antiarrhythmic agents. The remaining patient with recurrent life-threatening ventricular tachycardia underwent partial resection of a large left ventricular fibroma; no further episodes of ventricular tachycardia were observed postoperatively. Only one of the 15 patients with cardiac rhabdomyomas required antiarrhythmic therapy. None of the nine patients prospectively followed developed arrhythmias requiring therapy. The present data underline the fact that infants and children with cardiac tumours rarely need surgery because of arrhythmia resistant to medical treatment. As far as possible, conservative management is indicated in infants with suspected rhabdomyomas.