Blood transfusion has been widely touted as a useful adjunct to radiation therapy. This is based on the premise that higher hemoglobin levels will increase tissue oxygenation thereby improving the efficacy of therapeutic radiation. Consequently, it has become common clinical practice to transfuse patients undergoing radiotherapy to arbitrary hemoglobin levels. There has been extensive research evaluating the use of transfusion with radiotherapy. Animal data are divided regarding the radiobiologic benefits of transfusion. Clinical experience has established that anemia is a poor prognostic indicator in patients receiving radiotherapy. However, clinical trials have not clearly shown that correction of anemia by transfusion improves the outcomes. This may be explained by adverse effects of transfusion on tumor immunobiology. In addition to the well-known risks of infectious disease, alloimmunization, and allergic reactions, homologous blood has been shown to be immunosuppressive. This immunosuppression has been associated with decreased survival as well as increased rates of recurrence in the treatment of a variety of malignancies, thus making homologous transfusion particularly hazardous for the tumor-bearing patient. This suggests that routine use of transfusion to support patients receiving radiotherapy must be reappraised, and alternative methods of increasing hemoglobin, such as recombinant human erythropoietin (r-HuEPO), need to be considered. r-HuEPO represents a unique opportunity to assess the value of increasing hemoglobin mass without the confounding variable of immunosuppression. Consequently, a prospective clinical trial utilizing r-HuEPO in patients undergoing radiotherapy seems warranted.