Serum erythropoietin (EPO) levels were determined by the recombigen EPO RIA kit (DPC) in normal subjects and patients with renal dysfunction, diabetes mellitus, hypothyroidism and a variety of hematological disorders. Mean (+/- SD) serum EPO levels were 18.6 +/- 5.6 mU/ml in 180 normal subjects and no sex difference was obtained. Serum EPO levels in older subjects were slightly greater than those in younger subjects. There was a negative correlation between serum EPO levels and Ht values in anemic patients with normal renal function, whereas serum EPO levels were within the normal range in anemic patients with renal disorders, suggesting that serum EPO levels were relatively low in patients with chronic renal failure. Serum EPO levels were rather increased in patients with diabetes mellitus and hypothyroidism. High serum EPO levels were obtained in patients with a variety of hematological disorders such as acute leukemia, multiple myeloma, myelodysplasia syndrome, aplastic anemia and pure red cell aplasia. In a patient with pure red cell aplasia treated with glucocorticoids, serum EPO levels were lowered before anemia was recovered and reticulocytes were increased. These findings indicate that measurement of serum EPO levels are useful for not only differential diagnosis of anemia but also clinical evaluation of the treatment.