Muscle biopsy findings in two patients with Rett syndrome are reported from the Departments of Obstetrics/Gynecology, Pediatrics, and Pathology, Medical College of Ohio, Toledo, OH. Muscle biopsy was performed at 32 months of age and at 3 years 7 months of age. Light microscopy revealed fibers of uniform size with normal histochemistry. Electron microscopy revealed mitochondrial alterations including distention, vacuolation, and membranous changes. 
COMMENT. Abnormal mitochondria have been reported previously in the muscle biopsies of two patients with Rett syndrome . The findings presented in patients with Rett syndrome did not correspond to the typical “ragged red” fibers found in mitochondrial myopathies. There are no biochemical or pathological findings specific to Rett syndrome but further studies of mitochondrial functioning in muscle may be warranted.