The clinical and neurophysiological features of 18 cases of chronic asymmetrical spinal muscular atrophy are described. These were patients presenting with asymmetrical neurogenic atrophy involving one or more limbs who had no evidence of pyramidal tract dysfunction after 3 or more years of symptoms. There were twice as many males as females and the mean age of onset of the disorder was about 32 years. None of the patients had bulbar involvement. The tendon reflexes tended to be depressed. The distribution of muscle weakness in the limbs was very variable, and only slowly progressive. In 5 cases symptoms and signs were confined to the hands and forearms. Motor nerve conduction velocities to wasted muscles were slightly reduced but there was no evidence of generalised neuropathy. A diagnosis of chronic asymmetrical spinal muscular atrophy, as opposed to that of classical motor neurone disease, is favoured by an age of onset under 40 years, an absence of pyramidal signs or bulbar involvement after 3 years or more of symptoms, and depressed or absent tendon reflexes. The 2 conditions appear to be clinically distinct and prognosis is considerably better in chronic asymmetrical spinal muscular atrophy. The aetiology of this condition in unknown; it may be of relevance that 2 patients in this series had close relatives with Werdnig-Hoffmann disease.