Intrauterine growth retardation, microcephaly, and developmental delay in two first cousins lead to the recognition of phenylketonuria (PKU) in their mothers, 24- and 23 year-old sisters with blood phenylalanine concentrations of approx. 1.2 mmol/l who had never been treated and had no overt mental retardation. Both mothers were shown to be homozygous for a point mutation leading to an Arg-to-Gln substitution at codon 261 of the phenylalanine hydroxylase gene, a mutation which has been recently identified and tentatively associated with a mild variant of PKU. Our observation suggests that homozygosity for the Arg-261-Gln mutation can indeed result in "mild" PKU with little or perhaps no mental retardation, but also indicates that in such women, who may go unrecognized if not screened for, blood phenylalanine is elevated enough to cause the maternal PKU syndrome in their offspring.