The second case of an unusual motor neuron disease in an infant is described. The disease shows primarily an involvement of motor neurons, less of the proprioceptive and the extrapyramidal systems. The most prominent change is the vacuolization of many ganglion cells with a secondary denervation atrophy of muscle. Identical changes, but slightly more severe, were seen in our first case. Clinically and morphologically, the disease does not resemble the infantile spinal muscular atrophy of Werdnig-Hoffmann. The etiology is unknown.