We investigated erythropoietin (Epo) response in a cohort of diabetic patients with various types of anemia to approach the pathogenesis of some cases of “unexplained” anemia encountered among diabetics. Serum Epo levels were determined totally in 747 evaluable subjects with normal renal and hepatic function, of whom 694 had anemia. Among anemic patients, 237 were diabetics, while among the 53 nonanemic persons, there were also 21 diabetics. Diabetic and nondiabetic subjects were uniformly balanced in relation to their demographic features and were categorized according to the etiology of their anemia. Hemoglobin (Hb) did not differ between diabetic and nondiabetic subjects in all the etiological groups and in the whole population. Diabetic patients had significantly lower serum Epo levels as compared to nondiabetics (36.5±61 vs 69.4±191 IU/ml, p<0.0001), and this was true for all etiologic groups of anemia with the exception of patients with myeloproliferative disorders and those with megaloblastic anemia. The natural logarithmic (ln)–Epo×Hb component was used as an index of response to anemia and was found to be significantly decreased in almost all subgroups of diabetic patients. Serum Epo levels were also negatively correlated with the percentage of glycosylated Hb, HbA1_C (r=−0.446), and the correlation was stronger with the ln of serum Epo (r=−0.638, p<0.001). Inappropriately low serum Epo level is a uniform feature in patients with type II diabetes mellitus and may represent a constitutive blunted response to anemia or an altered metabolic rate of Epo, probably as a result of abnormal glycosylation of the cytokine.