Immunocytochemical analysis utilising antibody to neural cell adhesion molecule (N-CAM) was carried out on skeletal muscle biopsies from patients with childhood spinal muscular atrophy. Children with both Werdnig-Hoffmann and Kugelberg-Welander disease showed positive N-CAM reactivity. There were however differences in the N-CAM expression profiles in these two sets of patients. All myofibres were positive for N-CAM in the Werdnig-Hoffmann patients. This included both the normal sized fibres and the atrophic fibres. In contrast only the atrophic fibres were positive in the Kugelberg-Welander patients. No reactivity was found associated with the large hypertrophic fibres. It is likely that in the Werdnig-Hoffmann patients the positive N-CAM reactivity reflects unstable innervation of myofibres that had been previously innervated. A similar mechanism may operate in the Kugelberg-Welander patients, but the innervation of the hypertrophic fibres is more stable as they are able to repress N-CAM expression. These results contrast with a lack of N-CAM expression found previously on muscle biopsies from adults with denervation disease.