The neuropathological examination of the brain of a 4-month-old girl with Aicardi syndrome (infantile spasms, chorioretinal lacunae, psychomotor retardation, agenesis of the corpus callosum and vertebral anomalies) showed agenesis of the corpus callosum, agenesis of the anterior commissure and abnormal orientation of the hippocampal formation, in addition to periventricular cerebral nodules and extensive non-laminated cortical polymicrogyria of the fronto-parietal regions and gyrus cinguli of both hemispheres. A non-communicating medial supracollicular cyst, racemose cysts of the choroid plexus, underdevelopment of the inferior cerebellar vermis tonsils were also observed. The study of the polymicrogyric cortex with the Golgi method revealed an abnormal orientation of neurons; but the different varieties of cells were located at the depth corresponding to that were they are normally found. The Golgy study of the periventricular nodules disclosed the presence of different types of cortex-like neurons. On the basis of these data it may be suggested that polymicrogyria is not a migratory disturbance, but rather the result of a partial necrosis of the cortical mantle occurring before the 5th month of foetal life.