Retarded mental and motor development was observed during the first year of life of a full-term female who had been delivered with difficulty. Generalized psychomotor seizures and dysplastic "acromegalic" facial changes began to develop when she was 8 years old, and generalized amyotrophy developed over the next several years. The course was generally progressive, and she died at age 31. A variety of clinical laboratory studies were nondiagnostic. Autopsy findings showed normal brain weight and normal-appearing cerebral hemispheres, a peculiar atrophy of the inferior portion of the cerebellar hemispheres, a demyelinating process in the dorsal columns of the cervical and thoracic spinal cord, and lateral column and motor neuron degeneration. The pathogenesis of the syndrome is undetermined.