Unlayered polymicrogyria was analyzed in four patients with established lesions and in one 19- to 20-week-old fetus with lesions in a formative stage whose mother had suffered a serious accident two weeks before. Polymicrogyria occurred at the banks of porencephalic, sylvian clefts in three cases, and bilaterally in the watershed areas of the parieto-occipital lobes in a fourth case. Periventricular neuronal heteropias were found in these cases. Cortical lesions in the fetus were distributed along the watershed zones of the middle cerebral artery. Serial sections revealed that the appearance of microgyria was the result of radial tissue cleavage of the cerebral cortex, as shown by the increased numbers of blood vessels and astrocytes forming a tissue scar. Golgi studies disclosed that the different neuronal types were positioned at the apropriate cortical depths in the microgyric cortex. On the other hand, heterotopic nodules were composed of pyramidal and non-pyramidal neurons usually found in the upper cortical layers in the normal cortex. These features indicate that unlayered polymicrogyria is produced by circulatory failure occurring before the end of the period of neuroblast migration to the cortical plate. Circulatory failure in the radial and unbranched arteries that penetrate from the meningeal surface and vascularize the cerebral cortex at midgestation may result in radial tissue necrosis of the cortical mantle, whereas failure in the distal, terminal territories of these blood vessels may damage radial glial fibres and impair the last migration of neuroblasts. The particular morphology of this cortical abnormality finally depends on the imbalance in the tangential growth of adjoining cortical areas variably destroyed by tissue necrosis.