Acute or chronic anemia, a common complication in cancer patients, is associated with the development of tumor hypoxia. It has been shown in several trials that decreased hemoglobin (Hb) levels inducing tumor hypoxia adversely impact radiotherapy or combined radiochemotherapy outcome. For example, pre- and posttreatment Hb levels can be predicitive factors for the outcome of radiotherapy, locoregional tumor control, and survival. Two strategies have been established to correct cancer-related anemia and improve radiotherapy outcome: immediate increase of Hb levels with transfusions, or treatment with recombinant human erythropoietin (r-HuEPO, epoetin alfa), which slowly but steadily increases Hb levels to within normal range. As transfusions are associated with severe risks and adverse events, the use of epoetin alfa to treat preexisting anemia or prevent therapy-induced anemia represents an attractive strategy. The ability of epoetin alfa to maintain or to increase Hb levels in patients undergoing radiotherapy or radiochemotherapy have been shown in several studies in different tumor types. In addition to improving the results of radiotherapy and radiochemotherapy, anemia intervention with epoetin alfa may impact overall survival.