Two brothers with the chronic proximal form of spinal muscular atrophy (Kugelberg-Welander syndrome) were administered with ACTH and prednisone and a repeated remarkable and prompt improvement of the symptoms of this disease could be registered. Since similar experiences has recently been reported by other authors in hereditary sensory and motor neuropathy, a hypothesis is put forward of the possible participation of autoimmune mechanisms in the pathogenesis of some forms of hereditary so-called degenerative diseases of the peripheral motor neuron.