Surgery for atrial septal defect of the secundum type was performed in 84 patients over 40 years of age. Electrocardiograms were studied preoperatively and at a follow-up visit 74 months (range 22–174 months) after surgery. Preoperatively, atrial fibrillation was observed in 13% of the patients, complete right bundle branch block in 30%, and a secondary R wave (R’) in lead V<sub>1</sub> in 72%. A significant correlation was found between the amplitudes of S V<sub>6</sub> and R’V<sub>1</sub> and the right ventricular systolic pressure, whereas no significant correlation could be demonstrated between the shunt ratio and any of the QRS parameters. After successful surgery, the mean QRS axis in the frontal plane shifted from +99° to +47° (p < 0.01), and the following highly significant (p < 0.001) changes were noticed: increase of the amplitude of S V<sub>1’</sub> shortening of the QRS duration in V<sub>1’</sub> increase of the ratio QR’/QRS V<sub>1’</sub>, shortening of the duration of S V<sub>6’</sub> and diminution of the initial forces of the P wave in V<sub>1</sub>.