Anemia is a frequent complication of chronic kidney disease (CKD). Inadequate production of erythropoietin by the failing kidneys leads to decreased stimulation of the bone marrow to produce red blood cells (RBCs). Anemia of CKD develops early and worsens with progressive renal insufficiency. Although over 40% of patients with CKD are anemic, anemia in this population is under-recognized and undertreated. Of considerable importance, anemia is a risk factor for cardiovascular disease and is associated with higher rates of hospitalization and mortality. Despite the availability of erythropoiesis-stimulating proteins (ESPs) to stimulate RBC production in CKD patients, approximately three fourths of patients initiating dialysis have a hemoglobin <11 g/dL. The recognition of anemia of CKD begins with an estimation of glomerular filtration rate (GFR), which can be far lower than a normal serum creatinine might suggest, especially in the elderly and in those with poor nutrition and muscle mass. If GFR is <60 mL/min/1.73 m(2), hemoglobin should be checked. The anemia is diagnosed when the hemoglobin is <12 g/dL in a man or a postmenopausal woman, or <11 g/dL in a premenopausal woman. The cause of anemia should be investigated in these individuals; this can range from erythropoietin deficiency due to CKD, to deficiency of vitamin B(12) and/or folate, iron deficiency, blood loss, inflammation, malignancy, and aluminum intoxication. After other causes of anemia have been excluded, CKD is the most likely etiology, and it should be treated with an ESP. Currently, epoetin alfa and darbepoetin alfa are the only 2 ESPs approved for use in the United States. Extended dosing of ESP has potential advantages for the patient and may also improve resource utilization. Consequently, both agents have been tested for dosing at extended intervals. Adequate iron stores--defined as transferrin saturation >20% and ferritin >100 mg--as well as ESP administration are needed to produce an appropriate increase in hemoglobin. Poor response to treatment with ESP can be due to many factors, including presence of iron deficiency, inflammation, continued blood loss, and hemoglobinopathy.