Ventricular arrhythmias are common after repair of tetralogy of Fallot and are associated with late sudden death. This study examined the relation of ventricular arrhythmia during normal daily activities to the timing of repair, duration of follow up, and postoperative haemodynamic status. Forty eight hour ambulatory electrocardiographic monitoring was performed in 145 patients: 60 (aged 3 months to 46 years) had not yet undergone repair and 85 were followed from four to 22 (mean 14.6) years after repair. Cardiac catheterisation was performed in 47 (55%) one to 22 (mean 8.9) years after repair. Twelve (20%) of the uncorrected group had ventricular arrhythmia. The incidence increased significantly with age from 0% in patients aged less than 8 years to 58% in those aged greater than or equal to 16 years. In the corrected group 44% had ventricular arrhythmia. The incidence of arrhythmia was associated with older age at repair but not with postoperative haemodynamic status, duration of follow up, or era of surgery. It is concluded that ventricular arrhythmia is common in older patients with tetralogy of Fallot before repair and that during long term follow up of patients after repair the incidence of ventricular arrhythmia is influenced by the timing of surgery rather that the duration of postoperative follow up, era of surgery, or the haemodynamic result.