A description is given of a 25-year-old woman with unilateral paresis and atrophy of the muscles of the right arm and shoulder. Her symptoms started at the age of 20, took a progressive course from the distal to the proximal level, and became stationary about 4 years after onset. The paresis has the clinical characteristics of a spinal affection; the EMG also suggests a pathology at a medullary level, and muscle biopsy findings are consistent with a chronic affection of the peripheral motor neuron. The syndrome described by Hirayama et al. as ‘juvenile muscular atrophy of unilateral upper extremity’ is regarded as a useful clinical aid to temporary differentiation between such cases within the large group of atypical spinal muscular atrophies. It is to be expected that the pathological anatomical features will resemble those described by Marie and Foix as tephromalacia.