The analysis and description of the preoperative and postoperative vectorcardiograms in tetralogy of Fallot are presented. The spatial orientation of the maximum QRS vectors to the right, anteriorly, and inferiorly in group I is indicative of right ventricular hypertrophy. In group II patients, the vectorcardiogram showed evidence of increased electrical activity of the left ventricle.Vectorcardiographic patterns of complete right bundle-branch block were noted in nine patients in group III.