The severity of anemia in patients at different stages of the evolution of two tubulointerstitial nephropathies, Balkan endemic nephropathy and chronic pyelonephritis, was compared to clarify the previous observations that anemia appears earlier and is more severe in Balkan endemic nephropathy than in other renal diseases. The role of erythropoietin insufficiency as the cause of anemia in endemic nephropathy was studied as well. The severity of anemia increased with the impairment of renal function in endemic nephropathy and was similar to anemia in chronic pyelonephritis. However, in patients with endemic nephropathy at the initial stage of renal insufficiency significantly lower red cell concentrations were found compared with control subjects from the endemic region. In contrast, patients with pyelonephritis did not have decreased red cell concentrations at the early phase of their renal failure, suggesting that earlier appearance of anemia is characteristic for endemic nephropathy. To confirm this finding a study involving larger number of patients would be necessary. The serum erythropoietin levels, inappropriately low for the degree of anemia in patients with renal failure, were unrelated to the type of tubulointerstitial nephropathy.