Pure Red Cell Aplasia (PRCA), a rare type of anemia, may be associated with a loss of erythropoietin effectiveness in patients receiving recombinant erythropoietin. Post marketing data show that patients who later developed PRCA (with antibodies to erythropoietin) often demonstrated an initial positive response to treatment with recombinant erythropoietin which was followed by a loss of effectiveness. Some patients however have anti-erythropoietin antibodies without an apparent loss of effect of their recombinant erythropoietin treatment.. In this observational, international, multicenter study, patients with antibodies to erythropoietin who are taking an erythropoietin receptor agonist to treat or prevent anemia of any cause (for example, chronic kidney disease, cancer, human immunodeficiency virus) will be observed to see if they develop PRCA or a loss of effectiveness to erythropoietin and to measure how long treatment with a recombinant erythropoietin is effective. Patients will be followed for a total of 3 years. During the first 2 years, patients' erythropoietin antibody status (positive or negative), clinical progress and treatment or prevention of anemia, and outcome will be followed. The third year will consist of collection of follow-up information by phone or in person. During the study, no treatment will be specified and no medication will be supplied by the Sponsor of the study (Johnson and Johnson Pharmaceutical Research and Development, LLC). Patients will be treated by their individual physicians for their anemia. Study visits and blood-sampling for study-specific tests will be done at regular intervals throughout the first 2 years (every month to every 3 months, depending on test results). Safety will be assessed throughout the study by recording adverse events, including complications of red blood cell transfusions, any new diagnoses which are identified through the course of the patients' regular treatment and care for anemia, adverse events related to recombinant erythropoietin use or PRCA, complications of treatments or procedures for loss of effectiveness or PRCA and study-specific procedures. Patients may also participate in an optional portion of the study in which their genetic material will be analyzed to see if it contains something that would cause their immune system to make antibodies to erythropoietin. The study hypothesis is that a possible relationship exists between erythropoietin antibodies and the clinical course and outcome of patients with anemia in patients treated with recombinant erythropoietins.

Since the study is an observation of patients with anemia, 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 disease from their individual physicians.

Inclusion Criteria: - Male or female patients of any age - Receiving erythropoietin receptor agonist for any cause of anemia and having responded to this treatment for at least 3 months (response is defined by a rise in hemoglobin of up to 2 g/dL or more, a rise in hemoglobin of < = 2 g/dL for patients whose hemoglobin before treatment was >= 11 mg/dL, or a minimum 50% decrease in red blood cell transfusions since before treatment) - Patients whose blood tests are borderline or positive for antibodies to erythropoietin - Patients who have maintained a stable hemoglobin within a 3-month period that has not decreased more than 2 g/dL in any 1-month period without a cause that can be corrected ( for example iron, folate or vitamin B12 deficiency, ongoing blood loss, under active thyroid gland, uncontrolled overactive parathyroid gland) and with no increase in red blood cell transfusions - Patients for whom blood count information is available must have a count of reticulocytes (a type of immature red blood cell) >= 30 million/L

Exclusion Criteria: - Patients with a history of stem cell or bone marrow transplantation - Patients who have been treated with medication that decreases the ability of the immune system to function normally in the last 3 months - Patients who have shown a loss of effectiveness to treatment with an erythropoietin receptor agonist, defined as patients who initially responded to treatment for anemia [a rise in hemoglobin which lasted for a minimum of 3 months] followed by an unexplained decrease in hemoglobin (> = 2 g/dL) within a 1-month period