Microcephaly and mental retardation have been principal features of the fetal alcohol syndrome. This article describes the neuropathologic findings in four human neonates who were exposed to large quantities of ethanol at frequent intervals during gestation. The findings suggest that intrauterine exposure to ethanol can result in structural abnormalities of the brain. All four brains displayed similar malformations stemming from errors in migration of neuronal and glial elements. Hydrocephalus was one consequence of the malformations in two of the infants. Futhermore, the brain alterations may be the only distinct abnormality produced by in utero ethanol exposure. Only two of the four subjects were diagnosed as having the fetal alcohol syndrome from external criteria.