Tissues from three cases of hemimegalencephaly (HME) causing intractable seizures treated by cortical resection were studied using immunohistochemical, ultrastructural, and morphometric techniques. Severe cortical dysplasia was seen in all cases and included lesions best characterized as hemilissencephaly and polymicrogyria. Blurring of the cortex-white matter junction, the presence of large neuronal heterotopias, and neuronal cytomegaly were frequent observations. Immunohistochemical analysis demonstrated cellular colocalization of astrocytic markers glial fibrillary acidic protein and vimentin in one case of hemilissencephaly. Morphometric data showed significant increases over controls in neuronal profile area in all cases of HME. Neuronal cell density was increased significantly above controls in one of the cases. The study shows that HME results from severe cortical dysplasia which may be caused by multiple insults, manifest in one of several ways, and reflects abnormal or altered signals that regulate cortical morphogenesis.