PROGRESSIVE SPINAL MUSCULAR ATROPHY, juvenile proximal spinal muscular atrophy (Kugelberg-Welander), and infantile muscular atrophy (Werdnig-Hoffman) comprise a group of diseases by virtue of their pathological similarity. Chronic degeneration of the lower motor neurons and neurogenic atrophy of the skeletal muscle are common to all. The diseases differ in mode of inheritance, age of onset, distribution of muscular atrophy, and prognosis. Some investigators have considered them as a continuum of the same disease, underscoring the pathological similarity, while others have emphasized the clinical differences and prefer to regard them as distinct entities. This report describes 2 families in which 11 members, all male, were affected by an unusual, slowly progressive spinal and bulbar muscular atrophy. This disease, apparently in-herited as a sex-linked recessive trait, becomes manifest clinically in the fourth and Hth decades and initially involves proximal muscles. The proximal weakness gave a clinical picture similar to muscular dystrophy in some patients. A full description is made of the prepositus of each family, but only the more striking fea-tures or unusual aspects are mentioned in the other cases.