Ventricular arrhythmia is common after repair of tetralogy of Fallot. Such arrhythmia could be caused by damage at surgical repair or it could be part of the course of the disease. Fifty patients, 32 aged 1-7 years and 18 aged 13-43 years at surgical repair, underwent 24 hour electrocardiographic monitoring before and 2-75 (mean 44) months after repair. Before operation none of the younger group had appreciable ventricular arrhythmia, whereas ventricular arrhythmia was already present in eight (45%) of 18 patients in the older group. At follow up ventricular arrhythmia was present in four patients from the younger group and seven from the older group. Three of these four younger patients had had insertion of a conduit from the right ventricle to the pulmonary artery or reoperation. Six of the seven older patients had had appreciable ventricular arrhythmia before operation. New ventricular arrhythmia developed in only two (5%) of the 43 patients who had uncomplicated repair of tetralogy of Fallot. Thus the high frequency of ventricular arrhythmia after repair of tetralogy of Fallot cannot be attributed to the effect of operation alone because in the majority of patients it was already present before operation. Preoperative ventricular arrhythmia occurred almost exclusively in older patients; this suggests that early surgery may reduce the occurrence of this late complication.