Since birth a female child had been weak and hypotonic. At three months of age, head control was lacking; sucking and crying were poor. Four months later, there were more spontaneous movements and the girl was able to push herself up in prone position. Further motor improvement was noted at the age of 15 months. A 25-year-old brother of the patient's mother was very floppy during early childhood and has still some difficulties to swallow. Laboratory work-up showed elevated blood lactate and pyruvate levels, a mild hyperalaninemia and hyperalaninuria and an increased urinary excretion of dicarboxylic acids. Light and electron microscopy of a muscle biopsy disclosed a mitochondria-lipid-glycogen myopathy. Biochemical studies on a second muscle specimen revealed a combined deficiency of NADH-CoQ reductase and cytochrome c oxidase with a low carnitine level. There exists a considerable clinical and biochemical heterogeneity among the myopathies due to disturbances in the mitochondrial respiratory chain.