Zinc deficiency is common in developing country children, as food intakes are often low, foods from animal sources are infrequently used, the bioavailability of zinc from staple cereal-based diets is limited and zinc losses occur during recurring diarrheal illnesses. Zinc deficiency is associated with impairment in immunological and other defenses against infection and increased rates of serious infections. Due to limitations in currently used biochemical markers, supplementation trials in populations likely to be deficient provide a reliable means of assessing health consequences of zinc deficiency.