The anemia of chronic renal disease has been attributed primarily to a decrease in erythropoietin (EP) production. Results of this study show that measurable levels of plasma EP can be demonstrated in a majority of patients with end-stage renal failure who are undergoing long-term hemodialysis. These levels were not related to the type of renal disease, nor were they greatly affected by androgenic therapy or by nephrectomy. Although EP elevations in such patients were somewhat less than in nonuremic patients with comparable anemia, the presence of measurable EP levels suggests that impaired end-organ response may play a role in the anemia of chronic renal failure.