The diagnosis of cor pulmonale presents considerable difficulty in its earlier stages. Clinical symptoms are not available until the right heart fails and the only reliable sign is right ventricular hypertrophy, which is not easy to demonstrate. The purpose of this investigation is to evaluate electrocardiographic patterns of these patients as a more practical means of diagnosis. Sixty-seven patients with cor pulmonale are included in this study, which endeavors to point out degrees of correlation between various electrocardiographic patterns and clinical factors, such as stages of the disease, its severity, its etiology, and the anatomic changes of the right ventricle. It also evaluates the degrees of specificity of the various patterns encountered in these patients.