A 14-year-old girl of Ashkenazi Jewish background is described who has severe deficiency of hexosaminidase A. She presented with slowly progressive motor neuron degeneration and was found to have subtle deficiencies on neuropsychological testing, abnormal cortical evoked potentials and structural abnormalities of the cerebellum on CT scan. This patient extends the reported clinical findings for adolescents and suggests the possibility of similar abnormalities in others with neuromuscular deterioration.