Among breastfed infants, growth faltering in comparison with reference growth curves is common in both developing and developed countries. We performed a zinc supplementation trial in Paris, France, to find out whether such growth faltering is due to nutritional zinc deficiency. 57 breastfed infants aged 4-9 (mean 5.7) months were randomly assigned to receive either 5 mg zinc daily or a placebo for 3 months. Most of the infants were from low-income immigrant families and the majority were of African origin. Before supplementation there were no significant differences between the zinc and placebo groups in weight, length, or corresponding Z-scores for age. After 3 months' supplementation, the length-for-age Z-score had increased in the zinc group and fallen in the placebo group (+0.21 vs -0.13, p = 0.029). This difference was due mainly to greater linear growth of boys in the zinc than in the placebo group (6.0 vs 4.6 cm, p = 0.02). Weight gain was also significantly greater with zinc supplementation (1.64 vs 1.28 kg, p = 0.047). Among infants breastfed for longer than 4 months, decreases in growth velocity result partly from inadequate zinc intake.