Recombinant erythropoietins are effective in treating anemia caused by many diseases, including chronic renal disease. Pure red cell aplasia (PRCA, a rare form of anemia) and the development of antibodies to erythropoietin have been reported to occur infrequently in patients who are being treated with recombinant erythropoietin. The incidence of PRCA and antibody development is unknown. In this prospective, observational cohort study, specific information will be collected on patients with chronic renal disease who are receiving recombinant erythropoietin and their treatment course will be followed for up to 2 years. Patients will be treated by their individual physician for their chronic renal disease; no treatment or procedures will be specified and no medication will be given by the Sponsor of the study (Johnson and Johnson Pharmaceutical Research and Development, LLC). Upon entering the study, specific information will be collected regarding disease history and recombinant erythropoietin treatment. Every 3 months thereafter, progress information will be collected, including recombinant erythropoietin treatment, number of red blood cells and presence of any signs of PRCA development. If a confirmed or suspected diagnosis of PRCA is made, the date of diagnosis will also be recorded. Patients who develop PRCA will be offered the opportunity to enroll into another study specifically designed to follow PRCA patients for treatment, outcome and recovery. Additionally, blood samples will be collected at study entry and every 3 months for the purpose of screening/monitoring for antibodies to erythropoietin. If a patient's physician suspects PRCA or loss of effectiveness to erythropoietins at any other time during the study, an additional blood sample will be taken. Patients whose blood tests positive for antibodies to erythropoietin at any time will have blood sampled monthly until the tests are negative, at which time blood samples will be taken every 3 months until the completion of the study. Patients who discontinue erythropoietin will be followed only for an additional 12 months from the time erythropoietin is discontinued. All information will be collected in a confidential manner and identity of the patients will be protected. The results of this study will be combined with a similar study that does not collect blood samples to observe the number of patients who develop antibodies to erythropoietin. This study will initially enroll only patients with chronic renal disease who are being treated with a recombinant erythropoietin marketed by any of the Johnson and Johnson family of companies but may be expanded to enroll patients with other diseases and who have taken other recombinant erythropoietins. The study hypothesis is that the study will provide a framework for companion studies designed to investigate disease progression and population subgroup differences in further detail. Since the study is purely an observation of patients who may develop PRCA, no treatment will be required by the Sponsor and no medication will be supplied by the Sponsor. Patients will receive standard-of-care treatment for their chronic renal (or other) disease from their individual physicians.