In 80 patients who underwent heart surgery the incidence of electrocardiographic (ECG) changes after the operation was analysed. A precordial grid containing 24 leads and leads II, III and aVF was used. Electrocardiographic measurements were taken the day before the operation and again 5 days after the operation. New Q-waves were observed in 2 patients (2.5%) in the 24 precordial leads, in 2 patients in leads II, III and aVF, and in one patient in both precordial leads and leads II, III and aVF. New T-wave inversions were observed in 20 patients (25%) in the 24 precordial leads, in 5 patients (6%) in leads II, III and aVF, and in 3 patients in both precordial leads and leads II, III and aVF. A similar serum enzyme activity was observed both in patients developing Q-waves as well as T-wave inversions compared with cases in whom ECG changes did not appear.