The serum level of erythropoietin was measured in 31 patients with anemia secondary to chronic infection or malignancy and compared with erythropoietin levels in 23 patients with iron-deficiency anemia and 14 patients with primary hematopoietic diseases. Erythropoietin levels varied directly with the degree of anemia in patients with iron deficiency or primary hematopoietic disorders. There was no correlation of erythropoietin and the degree of anemia in patients with chronic infection or malignancy and the erythropoietin levels were significantly lower than in patients with iron deficiency or primary hematopoietic disease and the same degree of anemia. A major factor in the anemia of chronic disorders is a decrease in levels of erythropoietin.