Microgyria (polygria, polymicrogyria) has stimulated continued interest since its first description by Meschede [28]. Based on analysis of case material, available staining techniques, and known principles of human cortical development, subsequent investigators have proposed numerous theories to explain its pathogenesis. We have studied a case which cannot be fully explained by these previously proposed theories. In this case, four-layered microgyria is present bilaterally in middle cerebral artery distribution, but in one hemisphere, in the center of the malformed area, the deep acellular and cellular layers are replaced by radially aligned neurons extending ectopically into prospective white matter. Analysis of the findings in this case provide evidence that the recently described pathogenetic mechanism observed in a rat model of this malformation [15,16] is applicable to its formation in man.