Ultrastructural and biochemical alterations of muscle mitochondria are reported in two girls, aged 4 years, with Rett syndrome examined at the Institute of Neurological Sciences, University of Siena, Italy. Mitochondria were abnormally swollen and dumb-bell-shaped, and cytochrome c oxidase and NADH cytochrome c reductase were decreased in activity. The primary or secondary role of this mitochondrial pathology in the pathogenesis of the syndrome is undetermined. 
COMMENT. In contrast to the above, phosphorus magnetic resonance spectroscopy, proton MRS, muscle biopsies, and determination of pyruvate and lactate in plasma of 5 girls with Rett syndrome, examined at the John F Kennedy Institute, Glostrup, Denmark, failed to detect evidence for a mitochondrial disorder.