Maternal phenylketonuria (PKU) has serious consequences for the fetus as a result of elevated phenylalanine levels during pregnancy. An international survey of untreated maternal PKU pregnancies documented a 92% risk of mental retardation, 73% risk of microcephaly, 40% risk of low birth weight, and 12% risk of congenital heart disease in the offspring. Although the precise mechanism for fetal damage in maternal PKU is still unknown, it is clear that the fetus is harmed by the abnormal intrauterine environment. Treatment with a phenylalanine-restricted diet significantly reduces the risks, however, especially if initiated before pregnancy and maintained carefully throughout pregnancy. Nonetheless, despite the apparent simplicity of treatment, few women begin treatment before conception and maintain metabolic control during pregnancy. Therefore, even in treated pregnancies, developmental and neuropsychological outcome is extremely variable. In addition, genetic and psychosocial factors also contribute to the developmental and neuropsychological outcome in maternal PKU. This review summarizes the studies of children born to mothers with PKU and discusses factors affecting outcome. In addition, the similarities between maternal PKU, fetal alcohol syndrome, and other maternal conditions are presented. The predominant neuropsychological deficits that occur when the fetus is exposed to a teratogen are in the areas of language and memory. Thus, the hypothesis that maternal PKU represents a generic response to a teratogen is supported by these findings and by the characteristics of the developing brain.