Children with phenylketonuria (PKU), previously reported to show behavioral abnormalities and communicational deficits and to be extreme in activity patterns, were evaluated by a structured interaction procedure. PKU children, judged to be emotionally disturbed and inadequate in communication from independent impressions during psychological testing, performed more poorly than those PKU children judged nondisturbed and more adequate in communication under two of the three social stimulus interaction conditions. Children with PKU who manifested extreme patterns of activity were not significantly different in their interactional behavior from children with PKU who demonstrated normal activity levels. Differences in interaction behavior were obtained between children with PKU and normal control children similar on socioeconomic variables.