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The electrocardiographic conduction disturbances were evaluated retrospectively, in relation to prognosis, in 196 patients who underwent correction of tetralogy of Fallot. The follow-up was one to 20 years (mean 10). After surgery complete right bundle-branch block occurred in 187 patients (95%), right bundle-branch block and left axis deviation in 17 patients (9%), and progressive conduction defects, either left axis deviation or right bundle-branch block, developed during follow-up in 21 patients (11%). Nine patients (4.6%) died suddenly and two patients developed complete heart block late after the operation. Though late sudden death or complete heart block occurred in 19 per cent of patients with progressive conduction defects as opposed to 4 per cent of the group with stable conduction defects, the difference was not significant. Twenty-four hour ambulatory electrocardiographic monitoring was performed in 74 patients; 41 per cent had significant (Lown grade 2, 3, or 4) ventricular arrhythmias. The incidence of ventricular arrhythmia in the group with progressive conduction defects (80%) was significantly higher than in the group with stable conduction defects (30%). As occult arrhythmia may be the cause of sudden death, it is important to identify these patients.