SINCE the original descriptions of Wohlfart et al1and Kugelberg and Welander2there have been a number of reports of familial proximal neurogenic muscular atrophy. The principal findings of Kugelberg-Welander disease (K-W disease) consist of: (1) onset of muscle weakness in childhood or adolescence, (2) proximal muscle atrophy simulating muscular dystrophy, (3) very slow progression, (4) transmission as an autosomal recessive or dominant trait, and (5) neurogenic nature confirmed by electromyography or muscle biopsy. We present here two families, in which both male and female siblings are affected with this disease. We emphasize the remarkable difference in the clinical findings and course between affected male and female siblings. These findings may provide some suggestions in the nosological study of this disorder. Report of Cases Family I.— The proband (B) was a 19-year old unmarried woman. She was one of fraternal twins and her parents were cousins (Fig