Intraoperative hemodilution and retransfusion were used for blood replacement in 30 patients who underwent elective reconstruction of the abdominal aorta, thoracic aorta or femoropopliteal segment. During each operation, 1,000 to 2,000 milliliters of autologous blood were collected through an arterial cannula during induction of anesthesia and the initial stages of operative exposure. Each volume of shed blood was replaced with 1.5 volumes of colloid and crystalloid solutions to maintain a stable intravascular volume despite transient low hematocrit values--mean, 25.8 +/- 2.1 per cent--during the period of maximum blood loss in the surgical field. Collected blood was stored in standard citrate-phosphate-dextrose disposable units and was reinfused at the conclusion of the vascular procedure. Swan-Ganz pulmonary artery catheters were inserted in an initial study group of 15 patients. Serial blood hematocrit and coagulation studies and hemodynamic parameters were measured or calculated before and after induction of anesthesia, during hemodilution, after reinfusion of autologous blood and 24 hours after operation. Improvement in cardiac output and reduction in peripheral vascular resistance maintained adequate systemic oxygen transport during hemodilution. Transient dilution of coagulation factors was documented by abnormal prothrombin and partial thromboplastin times, but serious bleeding tendencies did not occur. Twenty-one of the 30 patients received no homologous blood. Considering the entire series of 30 patients, each required only 0.46 +/- 0.87, S.D., unit of homologous blood during operation and the postoperative period.