Analysis of the records of 15 patients with chronic cor pulmonale indicates that (1) the presence of chronic pulmonary disease is an indication to carry out a complete investigation of the patient's heart to establish the possible presence of cor pulmonale; (2) the outstanding clinical manifestations from the standpoint of symptoms and physical findings before frank heart failure appears are a poor index of the presence of this disease, but early historical and physical evidences of right ventricular strain, often neglected because of their minor character, are particularly helpful indicators for further investigation of the patient; (3) the addition of special investigation of the heart under the above circumstances with adequate electrocardiographic, fluoroscopic, and hemodynamic studies is necessary to establish the diagnosis of chronic cor pulmonale with any degree of accuracy in the stage before right sided heart failure appears, the stage most important in treatment.