SUMMARYHigher plasma free fatty acid concentration was observed in 15 of 33 young patients with generalized muscular atrophy. Hyperketonemia, which consisted mainly of the elevation of beta-hydroxybutyric acid, was observed in 6 of 10 patients with Duchenne-type dystrophy. Hyperketonemia was accompanied by the elevation of plasma free fatty acid level. No correlation was seen between plasma free fatty acid level and the severity of muscle involvement in the patient. This abnormal plasma free fatty acid level may be attributed to increased free fatty acid release from adipose tissue by as yet unrecognized causes. Consideration was given to the regulation of free fatty acid release in the myopathic patient.