Muscle biopsies of a 7-week-old girl with profound weakness of all but the ocular muscles, combined with hypotonia, hyporeflexia, hepatomegaly, macroglossia, myopathic electromyogram, and slight elevation of serum enzyme levels revealed mild glycogen and marked lipid and mitochondrial excess. Glycogen structure and anaerobic glycolysis were normal. Aldolase and pyruvate kinase levels were relatively low and the lactic dehydrogenase isoenzyme pattern resembled that of heart muscle. Subsequently the patient had normal intellectual and delayed motor development and her macroglossia disappeared. At 22 months pathologic alterations in muscle were strikingly improved and glycolytic enzyme levels and oxidation of labeled oleic acid and Krebs cycle intermediates were normal. Although still unidentified, the basic metabolic lesion is one that (1) affects mitochondria! morphology, (2) alters lipid and glycogen metabolism, (3) may improve with age, and (4) is compatible with a benign clinical course after early infancy.