The serum concentration of erythropoietin in 79 cases with various blood diseases, uremia, chronic obstructive pulmonary disease etc was determined. At comparable degrees of anemia, patients with myelodysplastic syndrome and aplastic anemia had the highest levels of erythropoietin in our study. The high level of erythropoietin titer in patients with aplastic anemia should be taken as the nom for renal synthesis and release of this hormone. The erythropoietin level in patients with uremic anemia was lower than the level in patients with anemia of other causes but still higher than that of the normal controls. Patients suffering from polycystic kidney disease with or without uremia had a high level of erythropoietin due to local hypoxia of remnant kidney tissue resulting from the pressure of cystic formation. Different methods are used to determine the erythropoietin level, which varies with the stage and etiology of the diseases. There are other stimulating or inhibitory factors of erythropoiesis when the assay is processed. Transfusion and administration of certain drugs also influence the growth of erythroid cells, thus the serum titers of erythropoietin differed markedly between patients at comparable hemoglobin concentration.