We estimated the efficacy of oral iron therapy during treatment with rhEPO in patients undergoing cardiac surgery who were contraindicated for autologous blood donation. Seventy-six patients were enrolled in this double-blind, placebo-controlled trial and assigned to the 2 treatment groups(5×500 U/kg body weight rhEPO or placebo intravenously over 14 d before surgery). During the treatment period all patients received 300 mg Fe2+ (iron glycine sulfate) orally per day. rhEPO therapy produced significant increases in hemoglobin concentration (Hb), reticulocyte count, hematocrit (Hct) and the hypochromic red blood cells (HRBC), and a decrease in transferrin saturation (41%) compared to the placebo group before surgery. However, the preoperative increase in HRBC was independent of the baseline ferritin and even correlated positively with the preoperative increase in Hct(r=0.47, p<0.01). In rhEPO patients there were inverse correlations between baseline serum iron and the preoperative increases in Hb (r=-0.39, p<0.05), Hct(r=-0.50, p<0.01) and HRBC (r=-0.53,p<0.001). With this treatment regimen the HRBC appear to reflect the degree of erythropoietic stimulation rather than functional iron deficiency. The preoperative increases in reticulocytes, HRBC and Hb/Hct in patients with ferritin <100 mg/l or transferrin saturation <16% showed no significant difference compared to their complementary groups. The preoperative decrease in storage iron and the inverse correlation between the baseline ferritin and the preoperative change in ferritin (r=-0.94,p<0.0001) in the rhEPO group indicate that the iron requirement for hemoglobin synthesis is probably covered by the breakdown of stored iron and an increase in the rate of absorption of orally administered Fe2+. Intravenous rhEPO treatment with 5×500 U/kg body weight in combination with 300 mg oral Fe2+/d given over 14 d before surgery is a suitable regimen to increase Hb by about 1.61 g/dl and Hct by 0.06.