Untreated maternal phenylketonuria (PKU) may result in nonphenylketonuric offspring with mental retardation, microcephaly, congenital heart disease, and low birth weight. We obtained information about 34 pregnancies in which dietary therapy was instituted to lower the concentration of phenylalanine in maternal blood in an attempt to avoid fetal damage. The outcomes varied from mental normality with no evidence of fetal effect to neonatal death due to congenital heart disease. Dietary therapy with control of the maternal biochemical abnormalities is not yet of proved efficacy in preventing these fetal effects. The available data tend to support initiation of dietary therapy prior to conception for best results, but the number of cases is small and points to the need for further research.