Few reports have dealt with the structural abnormalities shown by the neuropil in mentally retarded patients. This Golgi study describes the morphologic changes observed in a brain biopsy from the cerebral cortex of a patient with Bourneville's disease (epiloia). At the time this study was made, the patient was 12 years old and had had a long history of mental retardation and uncontrollable seizures. She, her father, and three other siblings had classic cutaneous signs of epiloia. A biopsy from the right frontal cortex was immediately fixed by immersion, was processed by both H & E and the Golgi method respectively, and examined by electron microscopy. The Golgi-stained material showed a marked fibrillary gliosis at the upper and lower cortical layers, as well as in the heterotopias; the presence of giant cells, closely resembling immature pyramidal cells, with short dendrites growing from their somata and bearing few spines; some other large cells having features compatible with astrocytes; spiral-like glial processes converging upon distorted apical dendrites; these and some other neuronoglial formations establishing specialized anatomical contacts; unorderly arrangement of small and large pyramids within an abnormally compact cortex; abnormal dendritic growths at the level of dendrite bifurcations; and several aberrant patterns of spine morphology, including the megaspine. This first application of the Golgi method to the study of neuropathologic features of epiloia suggests that a poorly developed neuronal circuitry led to the abnormal brain function observed in this case. The same anatomical substrate may occur in other cases of mental retardation.