Safety concerns surrounding the use of recombinant human erythropoietin (Epo) to treat anemia in cancer patients were raised after 2 recent clinical studies reported a worse survival outcome in patients who received epoetin a or epoetin b compared with patients who received placebo. Although those findings con- KEYWORDS: tumor, erythropoietin, receptor, proliferation, survival, angiogenesis, animal models, hypoxia.C oncerns that recombinant human erythropoietin (rHuEpo) may adversely affect the survival of cancer patients through promoting tumor growth recently were raised by 2 randomized clinical trials that used rHuEpo to prevent anemia in patients with breast or head and neck cancer. 1,2 In those trials, decreased survival and increased tumor progression were observed in patients who received epoetin a or epoetin b compared with patients who received placebo. These outcomes were in contrast to many clinical trials, which demonstrated that erythropoiesis-stimulating agents (ESAs) are safe and efficacious for the treatment of anemia in cancer patients. 3,4 These trials were criticized for poor study design and execution. 4 However, a number of hypotheses were proposed to explain the potential role of ESAs in promoting tumor proliferation and reduced survival including, stimulation of tumor expression of the erythropoietin (Epo) receptor (EpoR), tumor neovascularization, enhanced tumor oxygenation, and, most recently, the role of coadministered iron, 5 although that topic is not addressed herein.This review reflects conclusions from a comprehensive survey of the literature that investigated the expression and function of EpoR