To examine the effects of erythropoietin on the anemia of chronic renal disease and on the rate of renal deterioration, we administered recombinant human erythropoietin to 17 patients with anemia and progressive renal failure who did not yet require dialysis (serum creatinine level, 353 to 972 mumol per liter [4.0 to 11.0 mg per deciliter]). The dose of erythropoietin (50 to 150 units per kilogram of body weight) was adjusted according to the hematocrit response. In all 17 patients the anemia responded to erythropoietin. The median hematocrit increased from 0.27 to 0.37. The rate of the response depended on the initial erythropoietin dose and was similar to that observed in patients who were on dialysis. Hypertension was present in 14 patients before therapy, developed during therapy in 2 of the normotensive patients, and worsened in 9 patients, who required additional antihypertensive medications. The rate of the decline in renal function, as measured by serial determination of the reciprocal of the serum creatinine level, did not change significantly as the hematocrit rose (P = 0.78 by the paired t-test) during erythropoietin therapy. All the patients reported improvements in appetite, activity level, and sense of well-being. We conclude that erythropoietin therapy is effective in correcting the anemia of patients with progressive renal failure without affecting renal function, although it may be associated with an increase in blood pressure.