Nine cases of infantile spinal muscular atrophy were studied post-mortem. Their ages at death ranged from 5 months to 10 years. In all cases severe loss of anterior horn cells in the spinal cord and neurogenic muscular atrophy were characteristic of this disease. In 6 cases there was also loss of myelin in the posterior columns particularly affecting the lumbar contribution. Sensory ganglia, especially from the lumbar region, contained nodules of Nageotte, indicating sensory neuron degeneration. These sensory abnormalities were more severe in the longer surviving cases. It seems possible that sensory neuron degeneration occurs more commonly in Werdnig-Hoffmann disease than has previously been supposed but that it is less severe and develops more slowly than motor neuron degeneration.