This is a report on the natural history of 109 patients with infantile spinal muscular atrophy (SMA). All 18 children with the severe form died, as did five of the 52 with the intermediate form. Functional ability was more or less stable in the 47 living patients with the intermediate form, but 16 of the 39 with mild SMA lost the ability to walk. Ventilatory function was severely impaired in those with intermediate SMA: orthopaedic treatment for scoliosis in this group did not prevent the curves from worsening with age. Scoliosis was also severe in patients with mild SMA who had stopped walking: surgical treatment in six cases resulted in stable correction of the spine, with functional and cosmetic improvement and without impairment of respiratory function.