To evaluate the effect of long-term oral iron supplementation on growth, 250 children aged 6-71 mo were studied in a randomized double-blind controlled trial. The intervention group received 125 mg of ferrous gluconate (15 mg elemental iron) plus multivitamins (vitamins A, D and C); the comparison group received only multivitamins daily for 12 mo. Weight (kg) and height (cm) were measured every month. Eighty three percent of the children continued the treatment for one year. The weight increment over the 12-mo period was 1.35 +/- 0.65 kg (mean +/- SD) in the intervention group and 1.39 +/- 0.54 kg in the comparison group. The height increments were 6.01 +/- 1.47 and 6.18 +/- 1.58 cm in the intervention and comparison groups, respectively. Mean weight and height increments did not differ; in an analysis stratified according to different age and nutritional categories, they also did not differ between the two groups, indicating that long-term iron supplementation does not increase growth in children.