Two children, one homozygous and the other heterozygous for phenylketonuria, born of an untreated phenylketonuric mother were found to exhibit intrauterine growth retardation and persistent postnatal growth retardation. Microcephaly was present in both children at birth. Microcephaly, mental retardation, and growth retardation were present in the heterozygous child at 5 years of age and in the untreated homozygous child at 2 years of age. The possibility that the comparatively high phenylalanine level in the mother's blood and the concomitant abnormal metabolism had a detrimental effect on the child's prenatal growth and predetermined the rate of their postnatal physical as well as mental development, was discussed.