The impact of anemia on cancer patients undergoing chemotherapy is well established, but only recently has the prevalence of anemia in patients receiving radiotherapy received much attention. Many cancer patients present with anemia prior to radiotherapy, and even more experience anemia or a worsening of anemia at some point during treatment. However, the problem of anemia is often ignored because patients may experience only functional anemia, defined as a hemoglobin level less than 12 g/dl. Unless physiologic anemia (hemoglobin = 8 g/dl) is discovered, efforts to correct anemia are often not made. Because hemoglobin levels <12 g/dl seem to be associated with tumor hypoxia and poorer outcomes of radiotherapy in a number of patient populations, ignoring even modest anemia can result in decreased locoregional control, overall survival, and quality of life (QOL). Because increasing hemoglobin levels 1-2 g/dl is usually easily accomplished, there exists the potential for improving outcomes by paying greater attention to this problem. This article focuses on the prevalence of anemia, particularly functional anemia, and discusses the impact of anemia on locoregional control, overall survival, and QOL. INTRODUCTIONContinuing advances in oncology care, including new radiotherapy technologies and recent developments with multimodal therapies, have significantly improved clinical outcomes and quality of life (QOL) for many patients with cancer. However, the prevalence and impact of cancer-associated anemia is not widely appreciated. Emerging data demonstrate that cancer-related anemia is unexpectedly common in cancer patients receiving radiotherapy and/or chemotherapy and that even modest anemia (hemoglobin <12 g/dl) substantially impairs QOL. In addition, a growing body of literature suggests that hemoglobin concentrations of less than 12-13 g/dl during cancer therapy are associated with reduced local tumor control and decreased survival [1,2]. Although anemia of this magnitude can be easily managed, it is often overlooked or considered clinically insignificant. However, restoration of normal hemoglobin concentrations in patients undergoing radiotherapy has the potential to improve local tumor control and survival and produce sizable improvements in patient QOL.