Abstract The clinical, electromyographic and histological features are described in 15 patients suffering from juvenile benign spinal muscular atrophy. Two had had sibs who died in early childhood with similar but more severe symptoms and short comments on these 2 cases are included. The age at the onset of the disease varied from a few weeks to 15 years. The very variable progress of the disease, even within a single family, is emphasised and it is suggested that there is no clear demarcation between Werdnig-Hoffmann disease and the benign juvenile disorder (the Kugelberg-Welander syndrome). Certain cases even show affinities with adult progressive muscular atrophy. The finding of abnormal serum creatine kinase and aldolase activity and of ‘myopathic’ histological features in a small number of these cases is discussed and it is concluded that evidence does not justify the suggestion that there is a primary myopathic element in this disease.