Behavioral effects of chronic undernutrition were investigated. Subjects were 138 children, age 6-8, in rural Guatemala who had participated in the INCAP Longitudinal Study. Pre- and postnatal intakes of calorie supplements were the independent variables. Dependent measures were assessments of social interaction and affect. These measures were obtained by observing children in smallgroup activities with peers, and from individual cognitive tests. High calorie supplementation from birth to 2 years perdicted high levels of social involvement and both happy and angry affect at school-age, as well as moderate activity level, and low levels of dependency on adults, passive behavior, and anxious behavior. Relations were significant with SES and maternal supplementation controlled. Except for attentional measures, cognitive measures were not strongly predicted by supplement intake.