49 female blood donors with iron-deficiency anemia were treated with equal doses of iron either as carbonyl iron or ferrous sulfate in a randomized, double-blind fashion. The prevalence of side-effects was similar in the two groups. Mean values for hemoglobin concentration, mean corpuscular volume, corrected reticulocyte count, platelet count, serum iron, total iron-binding capacity, transferrin saturation or erythrocyte protoporphyrin did not differ significantly between the two groups throughout the study. After 16 weeks of therapy, the mean increase in hemoglobin iron was similar in both groups (p = 0.2). Estimates of net changes in total body iron suggested that the overall bioavailability of carbonyl iron was high, about 70% that of ferrous sulfate.