A controlled study was carried out to evaluate the effects of postoperative iron therapy on iron status in anemic children after cardiopulmonary bypass. The patients were 8 boys and 9 girls (mean age 6.5 years) who underwent elective closure of atrial septal defect, secundum type. On postoperative day 9, patients were randomly assigned to either iron supplementation with iron sulfate 5 mg/kg until day 56 or to a control group. Hemoglobin, reticulocytes, transferrin saturation, free erythrocyte protoporphyrin, and ferritin were measured, the final outcome measure being postoperative iron status on day 56. The treatment group showed higher transferrin saturations (33.5% versus 18.0%), smaller decreases in ferritin level (+3.0 versus--13.7 ng/ml), and a lower incidence of depleted iron stores (0/8 versus 5/9) than the control group (all data: P < 0.05). Anemic children after cardiopulmonary bypass for surgical repair of congenital heart disease thus benefit from iron supplementation within the first postoperative weeks.