BACKGROUND: Evidence for an effect of zinc supplementation on growth and morbidity in very young infants in developing countries is scarce and inconsistent. OBJECTIVE: We assessed the effect of zinc supplementation on growth and morbidity in poor Bangladeshi infants aged 4-24 wk. DESIGN: Infants from Dhaka slums were enrolled at 4 wk of age and randomly assigned to receive 5 mg elemental Zn/d (n = 152) or placebo (n = 149) until 24 wk of age. They were followed weekly for information on compliance and morbidity; anthropometric measurements were performed monthly. Serum zinc was assessed at baseline and at 24 wk of age. RESULTS: At 24 wk of age, serum zinc concentrations were higher in the zinc than in the placebo group (13.3 +/- 3.8 and 10.7 +/- 2.9 micro mol/L, respectively; P < 0.001). Significantly greater weight gains were observed in the zinc than in the placebo group for 43 infants who were zinc deficient (< 9.18 micro mol/L) at baseline (3.15 +/- 0.77 and 2.66 +/- 0.80 kg, respectively; P < 0.04). In the other infants, no significant differences were observed in mean weight and length gains during the study period. Zinc-deficient infants showed a reduced risk of incidence of acute lower respiratory infection after zinc supplementation (relative risk: 0.30; 95% CI: 0.10, 0.92); among the non-zinc-deficient infants there were no significant differences between treatment groups. CONCLUSIONS: Zinc-deficient Bangladeshi infants showed improvements in growth rate and a reduced incidence of acute lower respiratory infection after zinc supplementation. In infants with serum zinc concentrations > 9.18 micro mol/L, supplementation improved only biochemical zinc status.