We determined prospectively the diagnostic value of quantitative ultrasonography in detecting neuromuscular disorders in children. Ultrasonographic scans of four muscles were made in 36 children with symptoms or signs suggestive of neuromuscular disease, such as muscle weakness and hypotonia. The muscle thickness, ratio of muscle thickness to subcutaneous fat thickness, and echo intensity were determined in each muscle. The echo intensity was measured using computer-assisted gray-scale analysis. Thirteen of the 36 patients had a neuromuscular disorder (6 a myopathy and 7 a neuropathy). Differentiation between neuromuscular diseases and nonneuromuscular diseases could be made on the basis of echo intensities with a sensitivity of 92%, a specificity of 90%, a positive predictive value of 86%, and a negative predictive value of 95%. We conclude that computer-assisted quantitative analysis of muscle echo intensity is a reliable method to discriminate between neuromuscular and nonneuromuscular diseases in children.