We sought to determine the rate and extent of iron utilization after administration of intravenous iron dextran and to compare the efficacy of iron dextran preparations of differing molecular weight. We randomized patients to receive either a 500-mg dose of iron dextran molecular weight (MW) 267,000 (group A) or iron dextran MW 96,000 (group B) administered in five sequential 100-mg doses, and examined indices of iron status before and at weekly intervals up to 4 weeks later. Although mean iron utilization was greater in the nine group A patients (46.7% +/- 21.3%) than in the 11 group B patients (31.7% +/- 26.6%), the difference was not statistically significant (P = 0.19). Iron utilization in both groups was substantially incomplete. Changes in serum ferritin and hemoglobin did not differ between the treatments (P = 0.49 and P = 0.34, respectively). We conclude that iron utilization after iron dextran administration is substantial within the first week after completing a course of therapy, associated with stable iron indices after the first 2 weeks, and incomplete for at least the first 4 weeks. Degree of iron utilization appears independent of molecular weight within the range we examined.