To evaluate effects of Fe supplementation and sex on the prevalence of anaemia and Fe status in infants in South-East Asia, biochemical data from four parallel, randomized, double-blind trials with Fe and/or Zn supplementation in infants (n 2452) in Indonesia, Thailand and Vietnam was pooled. At recruitment (5 months of age), Hb concentrations were slightly but significantly lower in boy infants compared with girl infants (108.7 g/l v. 111.4 g/l, P = 0.04). At 11 months of age, boy infants not receiving Fe had significantly lower Hb (106.2 g/l v. 111.0 g/l, P < 0.001) and lower serum ferritin concentrations (14.3 microg/l v. 21.1 g/l, P < 0.001) than girl infants not receiving Fe. Consequently, boy infants had a relative risk of 1.6 (95% CI 1.3, 2.1) to be anaemic, and of 3.3 (95% CI 2.1, 5.0) for having Fe deficiency anaemia compared with girl infants. Fe supplementation significantly increased Hb concentrations in both boys and girls. There was no sex difference in Fe status in infants receiving Fe for 6 months. This study shows that the markedly higher risk for anaemia and Fe deficiency indicates higher Fe requirements in boy than in girl infants. In South-East Asia, standard infant feeding practices do not provide sufficient Fe to meet requirements of infants, especially boys. Current daily recommended intake for Fe in infancy is the same for boy and girl infants however. Our findings suggest that in especially the second half of infancy, Fe requirements for boy infants are approximately 0.9 mg/d higher than for girl infants.