The prevalence and factors favorising postoperative ventricular arrhythmias in 62 patients undergoing surgical repair of tetralogy of Fallot between 1971 and 1982 were analysed. Nineteen patients (31 %) had significant ventricular arrhythmias (Lown greater than or equal to 2) on Holter monitoring or exercise stress testing. The arrhythmia could only be recorded after stress testing in 5 patients. The patient's age at operation, the duration of follow-up and age at evaluation were significantly greater in the group with ventricular arrhythmias. However, age at operation was closely related to the other two parameters and a discriminating analysis showed that the age at operation was not a favorising factor for the occurrence of ventricular arrhythmias (p = 0.23), in contrast to the duration of follow-up (p = 0.0015) and age at evaluation (p = 0.0007). No relationship was found between ventricular arrhythmias and the following factors: previous anastomosis, outflow patch, necessity of a ventriculotomy or reoperation, presence of residual ventricular septal defect, postoperative systolic right ventricular pressure, severity of intraventricular conduction defects, and effort tolerance. These results suggest that after surgical repair of tetralogy of Fallot, the incidence of ventricular arrhythmias increases with time. The main problem is to identify patients with a high risk of sudden death, i.e. those with high degree ventricular arrhythmias and poor haemodynamic results of repair.