An exceptional type of cortical dysplasia is described in the brain of a 32-year-old woman who had received radiation therapy for a large pituitary adenoma 6 years before death. Markedly thickened gyri of the left inferior frontal, insular, and temporal cortex were found grossly. Microscopically, these gyri showed laminar disorganization and many unusually large and abnormally shaped ganglion cells. These neurons were heavily impregnated with silver preparations; were strongly reactive for neuron-specific enolase, synaptophysin, and neuronal cytoskeletal proteins (68- and 200-kd subunits of neurofilament protein, microtubule-associated protein 2, and tau); and ultrastructurally contained numerous perikaryal neurofilaments. Collectively, these findings suggest that the abnormally large, misshapen neurons contained excessive accumulations of cytoskeletal intermediate filaments. The present case and a similar one described in 1964 are the only two documented instances of neuronal gigantism apparently related to therapeutic irradiation of the brain.