A single fiber EMG (SFEMG) study was performed in 51 patients with idiopathic juvenile scoliosis of moderate degree (mean 23.8 degrees of Cobb), aged 7-18 years (mean 13.2 years). The findings in the extensor digitorum communis muscle (EDC) include a moderate but significant increase in fiber density (mean 2.02, +/- 0.21, P less than 0.001), a mild but significant (P less than 0.001) neuromuscular transmission abnormality (7.6% of fibers showed increased jitter and 4.5% intermittent blocking), and a moderately prolonged mean interspike interval (mean 0.98 msec, +/- 0.20, P less than 0.005) in EDC. Five of the patients had normal fiber density, 9 had a normal jitter study, and further 7 had a normal mean interspike interval. However only one had all the parameters normal. The paraspinal and intercostal muscles at the apex of the scoliotic curvature examined in some of the patients showed similar abnormalities. The study thus suggests the existence of a subclinical systemic neuromuscular disorder in nearly all of our patients with idiopathic scoliosis, which might have a pathogenetic significance.