The effects of a zinc supplement on growth velocity were assessed in a double-blind, pair-matched controlled study in 40 children with low growth percentiles. Participants were low-income Spanish-American children, 2 to 6 yr of age with heights below the 10th percentile and nutritional or biochemical evidence of zinc deficiency. After 1 yr, the mean height velocity of the zinc-supplemented children was slightly, but significantly (p less than 0.005), greater than that of control children. This effect was primarily due to a greater height achievement of the zinc-supplemented boys. Increases in height-for-age z-scores were also significant for the supplemented males (p less than 0.001) and for the combined sexes (p less than 0.05). This study indicates the existence of a growth-limiting syndrome of mild zinc deficiency in children.