The results of a food supplementation study conducted in four chronically malnourished rural villages in Guatemala were analyzed to determine possible effects on the study children's early mental and physical development. One of two food supplements, atole and fresco, were given to pregnant and lactating mothers and the infants born to them up to three years of age. The level of supplement received was noted and the subsequent course of the children's development observed. Food supplementation of pregnant mothers was found to correlate with higher weights of their babies at birth. No other major maternal variables--home diet, body height and/or weight, morbidity, obstetric characteristics, or socioeconomic status--could account for this association. At 36 months of age the children who had received a large amount of food supplementation showed a far lower prevalence of growth retardation than did those who received smaller quantities. Again, this correlation could not be attributed to any other major maternal variables. In regard to mental development, food supplementation was found to correlate with better performance in psychological tests beginning at six months of age. At 36 months the correlations were significant in five different tests. The results of this analysis have wide-ranging implications for public health. Given the recognized association between low birthweight and infant mortality, they point especially to the importance of maternal nutrition during pregnancy in programs aimed at reducing this serious problem.