In summary, anemia developing in a patient with cancer can be due to several different factors. A relative failure of erythropoiesis, in conjunction with a modestly shortened erythrocyte survival, is the most likely explanation for the anemia and can occur in patients with or without bone marrow invasion. Several theories have been proposed to explain the mechanism of limited red cell production in cancer. Internal iron starvation and cancer toxic factors have been widely implicated. Immunoglobulin inhibitors of erythropoiesis occur in the rare entity, pure red cell aplasia, which is sometimes associated with thymomas. Autoimmune hemolytic anemia and microangiopathic hemolytic anemia can also occur in patients with solid cancers, pointing out the need for a complete evaluation of anemia in any patient with recent-onset anemia. Successful treatment and prognostic implications of anemia in cancer is dependent on proper diagnosis.