This is a multicenter (study conducted at multiple sites), case-control (study that compare individuals with a disease or condition [cases] to a group of individuals without the disease or condition [controls] to determine the possible factor which increased disease incidence), retrospective (a study in which the participants are identified and then followed backward, as time passes) study. Retrospective risk factor data will be collected for control participants matched to the subset of participants in Protocol EPO-IMU-301 identified as having chronic kidney disease and anti-EPO antibody positive PRCA that began while the participant was receiving treatment with EPREX (index participants). For each index participant, up to 4 matched non-PRCA control participants with chronic kidney disease will be enrolled in this study. Approximately 600 control participants will be enrolled in this study. Control participants will be selected from the same site as the index participant and the data will be collected from the date closest to the reference date (loss of efficacy [drop in hemoglobin of greater than 2 g/dL/month] was first seen) that the control participant satisfies all study inclusion and exclusion criteria. The optional pharmacogenomic part (testing for polymorphisms and haploid types of the erythropoietin gene) will be recorded for the control participants who will sign the pharmacogenomics part of the study. No drug administration or treatment will be mandated by this study. Safety evaluation will include assessment of adverse events.