In a double-blind study, we compared the postoperative (post-op) blood loss in 161 children undergoing open heart surgery with cardiopulmonary bypass whose immediate post-op transfusion requirements were met with either very fresh whole blood (VFWB), 24- to 48-hour-old whole blood or reconstituted whole blood (packed red blood cells, fresh frozen plasma [FFP], and platelets). Assignment to treatment groups was not strictly random but dependent, in part, on the ability of families to provide directed donors for fresh blood. The three patient groups were comparable with respect to patient age, pre-op coagulation profiles (bleeding time, prothrombin time, activated partial thromboplastin time, platelet count, fibrin split products, fibrinogen, and platelet aggregation tests) difficulty of operative procedures and time spent on CPB. Mean 24-hour post-op blood loss in milliliters per kilogram was 50.9 +/- 9.3 in the VFWB group, 44.8 +/- 6.0 in the 24- to 48-hour-old group, and 74.2 +/- 8.9 in the reconstituted group (p = .03). When blood loss was compared in the 93 children less than 2 years of age, mean blood loss was 52.3 +/- 10.8 in the VFWB group, 51.7 +/- 7.4 in the 24- to 48-hour-old group, and 96.2 +/- 10.7 in the reconstituted group (P = .001). For subjects who had received reconstituted blood, 30- minute and 3-hour post-op platelet aggregation responses to adenosine diphosphate (10 mumol/L) and 30-minute aggregation response to epinephrine (2.5 mumol/L) were more depressed than in the VFWB and 24- to 48-hour groups (P less than .001, P = .005, and P = .02). Comparison of other post-op coagulation tests could not explain the increased blood loss in the reconstituted group. We conclude that the transfusion of less than 48 hours old whole blood is associated with significantly less post-op blood loss than the transfusion of packed red blood cells, FFP, and platelets in children under 2 years old who underwent complex cardiac surgery. The blood losses associated with the transfusion of VFWB and 24- to 48-hour-old blood are comparable and may be, in part, due to better functioning platelets.