The essentially indefinite storage life of previously frozen erythrocytes (PFE), combined with the virtual freedom from hepatitis, high 2,3-diphosphoglycerate (2,3-DPG) content, and low level of HL-A antigens, should make its use in open-heart surgery attractive. However, since the suspension medium for PFE is usually saline, the potential exists for creating a hemorrhagic diathesis by accentuating the dilution of plasma procoagulants by the pump prime. To test this possibility, we used PFE exclusively in transfusing a group of 13 open-heart surgery patients; they were given no plasma or platelets. A control group of 12 open-heart surgery patients were transfused with only shelf blood. Determination of prothrombin times (PT), partial thromboplastin times (PTT), platelets, and fibrinogen were done at various intervals. No clinically significant differences between the two groups were seen in any of these parameters at any interval, and there was no significant difference between the groups in amount of chest tube drainage or transfusions in the first 24 hours. It is concluded that most open-heart surgery can be safely performed exclusively with frozen blood.