To assess the effect of long-term administration of human recombinant erythropoietin (EPO) on renal function, 11 anemic children aged 1.4-17.2 years were followed for 10-61 (mean 31) months on treatment. During EPO therapy the mean hemoglobin rose from 8.1 to 11.1 g/dl at the last observation. The final maintenance dose ranged between 70 and 300 U/kg per week. The rate of deterioration of renal function was calculated by comparing the slope of the regression lines of reciprocal serum creatinine values (SCr) derived from a mean of 20 values per patient obtained over 8-50 (mean 29) months before and a mean of 24 SCR values during EPO therapy. The individual slopes improved after initiation of EPO therapy in all but 3 patients, but the mean change of slope (from -0.0521 to -0.0299) was not significant. The study suggests that in most children with predialysis chronic renal failure long-term administration of EPO is not associated with accelerated deterioration but rather with delayed deterioration of renal function.