To determine if high doses of oral iron could shorten the duration of therapy necessary to treat Fe deficiency anemia, high-dose Fe 600 mg three times per day (given as nontoxic carbonyl Fe) was compared with standard ferrous sulfate 60 mg Fe++ three times per day in a randomized, double-blind, 3-wk trial involving 36 female blood donors with mild Fe deficiency anemia. In animal studies, both forms of Fe have similar bioavailability when administered in equal amounts. High-dose carbonyl Fe was well tolerated with gastrointestinal side effects similar those observed with standard FeSO4 therapy. The 10-fold larger amount of Fe resulted in a mean 1.5-fold increase in estimated Fe absorption. Both regimens corrected anemia but neither replenished storage Fe. These results suggest that the principal advantage to the use of carbonyl Fe would derive from its safety rather than from the large doses that can be given.