This report describes part of a wider study on muscles from patients with adolescent idiopathic scoliosis. The aim of the study was to clarify if there exists a side-related pathology in the spinal musculature and if extraspinal muscles are abnormal in scoliosis patients. In scoliotic patients, both spinal and peripheral muscles showed frequent abnormalities when examined morphologically and histometrically by light and electron microscopy. Idiopathic scoliosis patients differed from the others. Morphologic pathology seemed worse on the concave side. A mild Type I fiber atrophy occurred in spinal muscles on the concave side and in the deitoids. A generalized tendency towards small myofibers was also noted. The findings suggest that there is a generalized specific neuromuscular disorder causing idiopathic scoliosis.