A wide spectrum of zinc deficiency states has been recognized in the weanling. Severe zinc deficiency affects most organ systems and causes a wide variety of clinical manifestations; these occur only in special circumstances. Zinc deficiency of intermediate severity has been reported in malnourished infants, in whom it may contribute to the clinical manifestations and limit the rate of recovery from protein energy malnutrition. Results of randomized, controlled studies of dietary zinc supplementation in Denver indicate that mild nutritional zinc deficiency is one cause of failure to thrive. Better laboratory indices for the determination of mild zinc deficiency are needed before the epidemiology of this syndrome can be understood. Quantitative data on zinc absorption from different weanling diets are required before it will be possible to determine the adequacy of zinc intake.