Hypoxia is now recognized as a major explanation of tumor radioresistance and is an important prognostic factor in head and neck and cervical cancers. Anemia, which had also a negative impact on cancer prognosis, increase tumor hypoxia. But we do not know actually if anemia has a direct role in tumor resistance or if it only reflect aggressiveness of the disease. Some trials have demonstrated that erythropoietin could increase the hemoglobin level during radiation courses. The only published randomised trial, performed in head and neck cancers, did not find any benefit for erythropoietin ; moreover, a deleterious effect was demonstrated. Possible explanations for this paradoxal result were discussed, particularly biological hypothesis and statistical bias.