The effect of iron supplementation on attending behavior of 96 1-y-old infants was assessed in a double-blind, randomized, controlled trial of iron dextran in Papua New Guinea. The treatment group received an injection of iron dextran at 2 mo; the controls received a placebo injection. Because many children had malarial parasitemia at testing, presence of malaria was used in the analysis. A significant interaction was found between iron and malaria infection on total fixation time: iron-supplemented groups and placebo-treated parasitemic children showed significantly higher total fixation scores than did placebo-treated aparasitemic children. Blood analysis of iron status showed similar results, with lowest iron status evident in the placebo-treated aparasitemic group. There was no effect of treatment on rate of habituation or dishabituation. Supplemental iron treatment has a significant effect on attention but the direction of the effect depends on the presence of malaria infection.