Electrocardiograms on 2, 123 Japanese (1, 611 rural individuals in Chiyoda and 512 urban individuals in Tokyo) over 40 years have been studied. The criteria for electrocardiographic classification by Blackburn et al. were used with some modification. Abnormalities were expressed in terms of percentage for each 10-year age group, and for 3 different blood pressure groups classified by the criteria suggested by WHO, for providing a means of relating incidence to age and hypertension. In Chiyoda, Q wave, left bundle branch block, prolonged PQ interval, and atrial fibrillation were lower in incidence. On the other hand, high voltage, some of ST and T changes and negative U were rather prevalent. The sex difference was frequently observed in incidence of several electrocardiographic findings.