1. 1. Hemodynamic and pulmonary function studies were evaluated in 15 patients with secondary polycythemia associated with chronic pulmonary emphysema. 2. 2. The studies were made before and after acute venesection and again after sufficient venesection to maintain hematocrits at nearly normal levels for periods of 5 to 6 weeks. 3. 3. Significant decreases in hematocrit, right atrial and right ventricular systolic, end-diastolic, and mean pressures, and significant increases in arterial oxygen saturation and tension, oxygen consumption, and arteriovenous oxygen difference were noted in measurements carried out 1 hour after the initial phlebotomy. 4. 4. In the total group, no changes occurred in pulmonary function studies after acute phlebotomy or repeat phlebotomies, or in hemodynamic studies after repeat phlebotomies. 5. 5. In patients with high end-diastolic right ventricular pressures at the initial catheterizations, acute phlebotomy was followed by decreases in the residual lung volume and functional residual capacity, although these had increased to the control values after 6 weeks. 6. 6. It is possible that the observed changes in O 2 gas studies and residual lung volumes may be related to changes in intrapulmonary blood volume or flow. 7. 7. All of the changes observed after repeated phlebotomies can be explained by the mechanical removal of red blood cells. 8. 8. There is no indication that polycythemia, of itself, is detrimental to the hemodynamic or pulmonary function status of these patients when heart failure has not supervened. 9. 9. Venesection has a therapeutic role in the management of patients with secondary polycythemia associated with chronic pulmonary emphysema. Immediate phlebotomy should be helpful in controlling heart failure, and repeat phlebotomies, although not altering the hemodynamic or pulmonary function status of these patients, may be indicated because of the reduction in the viscosity, and, therefore, presumably for the effect on the incidence of thromboembolic complications. It would appear that the utilization of repeated phlebotomies should be determined by the degree of polycythemia rather than by the pulmonary function status of the patient.