As the antitumor activity of radiation is mediated via its interaction with oxygen to form labile free radicals, the intratumoral oxygen level has an important influence on the ability of radiation therapy to kill malignant cells. By decreasing the oxygen-carrying capacity of the blood, anemia may result in tumor hypoxia and may have a negative influence on the outcome of radiotherapy for various malignancies, even for small tumors not normally assumed to be hypoxic. In addition, anemia also has a negative effect on the quality of life of cancer patients, as evidenced by worsening fatigue. As a high proportion (about 50%) of cancer patients undergoing radiotherapy are anemic prior to or during treatment, strategies to correct anemia and/or the resultant tumor hypoxia are increasingly being considered an important component of treatment. In particular, epoetin alfa (recombinant human erythropoietin), which has proved an effective and well-tolerated means of raising hemoglobin levels in anemic patients receiving radiotherapy, potentially could reverse the negative prognostic influence of a low hemoglobin in patients with certain malignancies. Radiation oncologists need to be aware of the possibility of anemia in cancer patients undergoing radiotherapy so that timely intervention can be instituted whenever anemia is diagnosed.