1. 1. A series of forty cases of constrictive pericarditis is analyzed and the subject reviewed in the light of this experience. 2. 2. One-quarter of the cases were still in the active stage of disease. The chief differences between these patients and those with chronic inactive pericarditis are emphasized. 3. 3. A paradoxical pulse, a dominant and sharp x descent in the jugular pulse, normal rhythm, more than slight enlargement of the heart shadow and persistent hydrothorax were noted more commonly in the group with active pericarditis. Atrial fibrillation, a relatively small heart and pericardial calcification strongly indicated inactive cases of long duration. An early third heart sound and a characteristic electrocardiographic pattern occurred with equal frequency in both groups. 4. 4. In differential diagnosis perhaps the only serious difficulty is in distinguishing constrictive pericarditis from cardiomyopathy of clinically obscure origin. Factors indicating cardiomyopathy are a conspicuous left ventricular cardiac impulse, a third heart sound falling at its usual time, mitral or tricuspid regurgitation, bundle branch block, electrocardiographic changes denoting left ventricular hypertrophy or necrosis and more than slight cardiac enlargement radiologically. 5. 5. In the absence of thromboembolic pulmonary hypertension, the left atrial pressure is usually 10 to 20 mm. Hg higher than the right in cases of cardiomyopathy, the divergence increasing with exercise; in chronic constrictive pericarditis the two atrial pressures usually remain similar under all conditions. The implication that cardiomyopathic heart failure is initially and predominantly left-sided is accepted. 6. 6. The cardiac output averages about 25 per cent lower and the arteriovenous oxygen difference 33 per cent higher in cardiomyopathy than in chronic constrictive pericarditis. The maintenance of a resting cardiac output well over 4 L./minute when there is atrial fibrillation is characteristic of constrictive pericarditis. 7. 7. The absence of reactive pulmonary hypertension in constrictive pericarditis despite resting left atrial pressures in the mitral stenotic range is noted. 8. 8. The operative mortality was 11 per cent in the twenty-seven cases treated surgically. The results were good or excellent in 82 per cent and poor or indifferent in 7 per cent. Reconstriction was not observed. Activity is no bar to successful surgical therapy; none of the seven patients with active pericarditis operated upon in this series died.