The purpose of this study is to estimate the incidence rate of pure red cell aplasia (PRCA; aplastic anemia) mediated by erythropoietin (EPO) antibodies in patients who are receiving subcutaneous (s.c.) epoetin alfa (polysorbate 80 formulation) for the treatment of anemia associated with chronic renal failure (CRF), and to compare this incidence rate to the incidence rate with s.c. exposure to other currently marketed recombinant erythropoietin products (epoetin alfa, epoetin beta, darbepoetin alfa), with adjustment of duration for which the drug is given to the patient. The study will also examine the impact of the pattern of using mixed s.c. exposure to multiple erythropoietin products occurring in this patients, and the impact of the time from which the treatment is started to the onset of PRCA.