We conducted a multiple-center joint study on the effects of recombinant human erythropoietin (rEPO) for predialysis patients. rEPO was intravenously administered to 42 predialysis patients (13 males and 29 females) with hematocrit (Ht) levels of less than 30%. The subjects were divided into group A (28 cases) in which rEPO was administered twice a week, and group B (14 cases) with rEPO administration once a week. The initial administration dosage was 6000IU/week. The Ht levels were 22.6 +/- 3.3% for group A and 23.2 +/- 2.7% for group B before the administration of rEPO, and increased to 31.0 +/- 4.0% and 27.7 +/- 3.7% respectively twelve weeks after initiating administration. The levels of effective improvement on anemia included 'markedly effective' in 17 cases (80.9%) and 'effective' in 2 cases (9.5%) in group A, and 'markedly effective' in 5 cases (41.7%) and 'effective' in 3 cases (25.0%) in group B. No significant change was seen in serum creatinine (Cr) levels during the study period. In the evaluation of renal function by reciprocal serum creatinine (1/Cr), a consistent tendency was not recognized; thus, suggesting that the rEPO administration had no effect on the renal function. No variation of blood pressure was seen. As far as side effects were concerned, headache and heavy headedness were recognized in four cases. There were, however, no cases in which the severity of the side effects dictated the discontinuation of the rEPO administration. In conclusion, rEPO was judged to be a safe and effective treatment for the anemia of predialysis patients.