Preoperative blood reservation for autologous blood transfusion generally causes anemia. We performed a double blind controlled study to determine the optimal dose of subcutaneous rHuEPO (recombinant human erythropoietin, KRN5702) for preventing preoperative anemia due to blood reservation. Patients received KRN5702 subcutaneously once a week in a doses of 12000, 24000 or 36000 IU by a double blind technique. After storage of 1200 ml of their own blood right before surgery, their hemoglobin (Hb) averaged about 1 and was about 0.5 g/dl lower than the level before administration of KRN5702 in doses of 12000 and 24000 IU, respectively. This fall was significant. In patients receiving KRN5702 in a dose of 36000 IU, the level of Hb rose instead of a fall; Hb immediately before surgery was 1.1% higher than that before administration which, however, was not significant. This elevation indicates a possibility of abnormal elevation of Hb at this dose. The mean Hb value right before surgery was significantly lower in patients receiving 12000 IU KRN5702 than in patients of the other two groups. The recovery rate of Hb was an indicator to reflect improvement of anemic conditions, and increased as the dose increased after the blood reservation. The rate in the 12000 IU group was significantly lower than that in the other two groups; there was not much difference between the other two rates. We estimated that to reserve 1200 ml of autologous blood, 24000 IU of KRN5702 is adequate but not excessive. One patient receiving 24000 IU showed side effects including an elevation of body temperature, rash, and edema.