Zinc deficiency places children in many low-income countries at increased risk of illness and death from infectious diseases. Randomized controlled trials of zinc supplementation provide the best estimate of this risk through demonstrated preventive benefits. In six of nine trials that evaluated prevention of diarrhea, significantly lower incidence of diarrhea occurred in the zinc group than in the controls; a pooled analysis demonstrated 18% (95% confidence interval, 7-28%) less diarrhea. In five trials, a lower rate of pneumonia infection was found in the zinc-supplemented groups, and there was some indication of a preventive effect in three trials with a clinical malaria outcome. Zinc was also found to have a therapeutic benefit in seven trials of acute diarrhea and five of persistent diarrhea. Studies to evaluate the effect of zinc supplementation on mortality are under way, but a recently published study from India identified a 68% reduction in mortality in small-for-gestational-age term infants that were supplemented with zinc from 1 to 9 mo of age. The important effects of zinc deficiency are now clear, and nutrition programs should address this prevalent problem.