A detailed neuropathologic examination of the brain of a child with the typical epidermal nevus syndrome revealed a primary disturbance of development of the brain. The developmental disturbance consisted of an abnormal or incomplete migration of neurons to form the cerebral and cerebellar cortices. The normal architectonic pattern of the cortical layer formation of the cerebrum and cerebellum was also disturbed. In addition, there was an abnormality in the proliferate activity of astroglial cells resulting in gliomatosis cerebri. It is suggested that the basic abnormality underlying various neurologic derangements in epidermal nevus syndrome is the result of a defect in specific development events, such as neuronal migration and cortical differentiation.