We evaluated the brain of a 4-month-old male infant whose mother had inadequately controlled maternal phenylketonuria (MPKU). At autopsy his brain was normally developed but underweight. We found ventriculomegaly, hypoplastic cerebral white matter, and delay of myelination in late myelinating tracts without white matter astrocytosis and without chronic lesions in any gray matter structure. We compared the development of the infant's white matter tracts with published data on infant myelination. Congenital heart disease complicated the case. Abnormalities in developmental white matter may account for neurological abnormalities in infants with MPKU.