Quantitative assessment of cultured endothelial progenitor cells (EPCs) from 11 patients with renal anemia during rhEPO treatment. Administration of rhEPO clearly resulted in a marked increase in total EPC number within 8 weeks of therapy. *P < .01, comparison of EPC numbers at week 2, 4, 6, and 8 versus baseline. We observed no difference in the EPC response to rhEPO therapy in those patients who had received a weekly rhEPO dose above 5000 IU (n = 6) and in those patients who received an rhEPO dose below 5000 IU per week (n = 5).