Duchenne muscular dystrophy (DMD) is one of a range of muscular dystrophies caused by abnormalities of the short arm of the X chromosome (Xp21), which often cause mental retardation in addition to progressive muscular weakness. Normal dystrophin expression is lacking in both skeletal muscle and brain of affected subjects. Phosphorus-31 magnetic resonance spectroscopy has shown several abnormalities in skeletal muscle in DMD. We looked for similar abnormalities in brain in patients with DMD and related the findings to neuropsychological test results. We studied by magnetic resonance spectroscopy 19 boys (aged 76-167 months) diagnosed as having DMD and 19 control boys of similar age (87-135 months). Intelligence quotient (IQ) was assessed with the Wechsler Intelligence Scale for children. The DMD patients had significantly higher values than the controls in the brain ratios of inorganic phosphate to adenosine triphosphate (mean 0.53 [SD 0.21] vs 0.36 [0.09], p = 0.003), to phosphomonoesters (0.40 [0.07] vs 0.29 [0.07], p = 0.0001), and to phosphocreatine (0.44 [0.10] vs 0.37 [0.08], p = 0.02). There were significant differences between the DMD patients and the controls in full-scale IQ (76 [16] vs 101 [16], p = 0.0001), performance IQ (78 [17] vs 94 [14], p = 0.003), and verbal IQ (78 [17] vs 106 [17], p = 0.0001). These altered metabolite ratios parallel the findings in dystrophic muscle and suggest bioenergetic similarities in tissues that lack dystrophin.