Abstract 1. 1. The methods of catheterization of the heart and the pulmonary vessels make it practical, for the first time, to investigate the pressure and flow phenomena in the right side of the heart and in the pulmonary vascular system of patients with constrictive pericarditis. Six such patients have been studied by these methods. 2. 2. Every patient showed an elevation of pulmonary “capillary” pressure (which reflects pulmonary venous pressure). This demonstration of pulmonary congestion is interpreted as indicating that involvement of the left ventricle plays a more important and sinister role in the functional changes associated with pericardial constriction than has been demonstrated previously. 3. 3. Every patient also showed an elevation of pressure in peripheral veins and in right atrium, and pressure measurements in the right ventricle and pulmonary artery which indicate a reduced ability of the right ventricle to contribute to the forward movement of blood. These observations confirm and emphasize the previously recognized involvement of the right ventricle. 4. 4. In every patient before operation the mean pulmonary capillary pressure was approximately equal to the peripheral venous pressure. 5. 5. No patient exhibited pressure changes pointing to physiologically significant obstruction in great veins or auricles. 6. 6. The limitation of stroke volume and cardiac output per minute in constrictive pericarditis previously demonstrated by other methods is confirmed by these studies. 7. 7. Consideration of the course of events after operation suggests that myocardial atrophy, myocardial fibrosis, and incomplete release of the ventricles may all play a role in the slow, and in most cases, incomplete return to normal dynamics after operation. 8. 8. The specific therapeutic implications of this study are: (A) the objective of surgery is the adequate release of both ventricles; (B) no indications have been found for the decortication of great veins or auricles, and (C) the early use of antibiotics in acute tuberculous pericarditis may minimize myocardial fibrosis and may permit operative treatment before myocardial atrophy is severe. 9. 9. Finally, the fact that improvement after operation is often slow and often incomplete in terms of objective measurement should not lead physicians to ignore the more important fact that a well-planned operation for constrictive pericarditis has in the past usually made the difference between invalidism and activity. The studies reported in this paper may be expected to make future operations for this disorder even more effective.