The efficacy of iron supplementation for iron-deficient subjects is in no doubt. However, the assumption that iron supplementation of iron-replete subjects is harmless may not be valid. We have studied the effect of iron supplementation on growth rate in 47 iron-sufficient young children (12-18 months) in Indonesia. The children were randomly assigned either ferrous sulphate (3 mg/kg daily) or placebo every day for 4 months. Before treatment the length, weight, and arm circumference of the two groups were similar. During the 4 months of supplementation the rate of weight gain was significantly greater in the placebo group than in the iron-supplemented group (0.106 [SE 0.010] vs 0.070 [0.011] kg every 2 weeks, p = 0.02). The rates of gain in length and arm circumference did not differ significantly by treatment. There were no differences between the groups in rates of respiratory and gastrointestinal infections. These results suggest that iron supplementation of iron-replete children may retard their growth.