In children in developing countries, zinc deficiency may be common and associated with immune impairment and increased risk of serious infectious diseases such as diarrhea, pneumonia, and malaria. Studies have evaluated the therapeutic effects of zinc supplementation during acute or persistent diarrhea. In studies of acute diarrhea, the illness duration has been found to be 9-23% shorter in zinc-supplemented than in control children. Diarrhea was also less severe in zinc-supplemented children. In studies of persistent diarrhea, the effect sizes were similar but were often not statistically significant, perhaps because of the small number of children participating in these studies. Trials that provided continuous daily zinc supplementation for 5-15 mo evaluated effects on the incidence of diarrhea and in some studies acute lower respiratory infections and malaria. The reduction in the incidence of diarrhea in the zinc-supplemented group in these studies ranged from 8% to 45%. A study that gave 2 wk of zinc supplementation found preventive effects against diarrhea for the 3 mo of surveillance. More limited data also suggest that the incidence of acute lower respiratory infection and clinical attacks of malaria may also be reduced by zinc supplementation. If these results are confirmed by meta-analysis of the existing trials and additional research, improvement of zinc nutriture should become a priority intervention to reduce the high burden of serious infectious disease in children in developing countries.