Described are certain clinical and morphologic features of one patient with acute, another with subacute, and one with chronic cor pulmonale. All 3 had evidence of severe pulmonary hypertension. The patient with acute cor pulmonale 4 days after coronary bypass for unstable angina pectoris suddenly developed severe breathlessness with cyanosis and had fatal cardiac arrest and necropsy disclosed massive pulmonary embolism. The patient with subacute cor pulmonale had severe right-sided heart failure for 5 weeks and necropsy disclosed microscopic-sized neoplastic pulmonary emboli from a gastric carcinoma without parenchymal pulmonary metastases. The patient with chronic cor pulmonale had evidence of right-sided heart failure for years, the result of primary or idiopathic pulmonary hypertension almost certainly present from birth because the pattern of elastic fibers in the pulmonary trunk was that seen in newborns where the pressure in the pulmonary trunk and ascending aorta are similar. The patient with chronic cor pulmonale had plexiform pulmonary lesions indicative of irreversible pulmonary hypertension. Neither the acute nor the subacute patient had chronic pulmonary vascular changes. All 3 patients had dilated right ventricular cavities and non-dilated left ventricular cavities and only the patient with chronic cor pulmonale had right ventricular hypertrophy.