Magnetic resonance imaging (MRI) scans were performed on 13 high-functioning male autistic subjects and 13 male nonautistic control subjects comparable in age and nonverbal IQ. Scans were rated for the presence of cerebral cortical malformations. Five autistic subjects had polymicrogyria, one had schizencephaly and macrogyria, and one had macrogyria. None of the control subjects had abnormalities of this type. These abnormalities result from a defect in the migration of neurons to the cerebral cortex during the first 6 months of gestation. The detection of these malformations by MRI, their pathogenesis, and the implications regarding the pathogenesis of autism are discussed.