Anemia is a common symptom in cancer patients. The relationship among anemia, cancer progression and clinical outcomes of cancer treatment is complex and much remains to be elucidated. One hypothesis for an etiological link is that anemia contributes to the development of tumor hypoxia, which in turn has been shown to induce proteomic and genomic changes within the tumor cells that ultimately favor malignant progression and treatment resistance. A substantial body of clinical data indicates that anemia can be a significant independent prognostic factor for treatment response and survival in cancer patients treated with chemotherapy or radiotherapy. In addition, studies have shown a correlation between anemia and decline in quality of life (QOL) in cancer patients. Of the many factors that impact cancer patients' QOL, fatigue has emerged as the most prevalent, troubling, under-recognized and under-treated of all symptoms experienced by anemic cancer patients. Systematic correction of anemia with appropriate supportive therapies prior to or during chemotherapy or radiotherapy may enhance patients' QOL.