It has been reported that serum levels of ketone bodies often elevate in patients with Duchenne muscular dystrophy (DMD). The cause of this abnormal metabolism, however, has not been elucidated. In this study, serum levels of ketone bodies were measured in the early morning in patients with DMD, congenital muscular dystrophy and cerebral palsy. Serum free fatty acids (FFA), glucose, and thickness of subcutaneous fat were also measured. Ketone bodies elevated in most DMD patients, but not elevated in patients with the other disorders. Hyperketonemia in the DMD patients was accompanied by elevation of the FFA level, but no correlation was seen between hyperketonemia and the creatine kinase level or the severity of muscle involvement. No differences were found between DMD patients and others in the levels of FFA, glucose and thickness of subcutaneous fat. These results indicate that DMD patients are more prone to ketosis than the others. The effect of biotin administration (2 mg/day) on hyperketonemia also was investigated in 11 patients with DMD. The levels of total ketone bodies decreased by biotin administration for 2 weeks. These data suggest that the utilization of FFA and ketone bodies may be impaired in DMD patients. Biotin treatment will be useful for improving the abnormal metabolism of ketone bodies in DMD patients.