We report an autopsy case of a 67-year-old man clinicogenetically diagnosed as having spinal muscular atrophy (SMA) type III (Kugelberg-Welander disease), showing slowly progressive muscle wasting and weakness of the extremities. His brother showed similar manifestations. Autopsy revealed neuronal loss and severe gliosis in the anterior horns of the spinal cord, a marked neurogenic change of skeletal muscles and mild degeneration of cardiomyocytes. Chromatolytic change was seen in the anterior horn, but not in the Clarke's and thalamic nuclei. The anterior spinal roots were atrophic, and there was loss of myelinated fibers with abundant glial bundles. In addition, degeneration was also observed in the posterior column and dentate nucleus. The pathological features were essentially similar to those of SMA I. Chronic change was prominent while acute change was mild in degree, corresponding to a very long clinical course.