Two siblings with infantile lactic acidosis and mitochondrial myopathy are reported from the Department of Pediatrics, Goteborg University; Ostra Hospital; Goteborg, Sweden. The first child, a girl, appeared healthy during the first four months of life. She was admitted at five months of age with feeding difficulties, vomiting and weight loss and muscular hypotonia. Her serum lactate concentration rose to 20 mmol/L (n:0.8-1.8 mmol/L), she developed edema, became comatose and died of circulatory failure eight days after admission. At autopsy, the heart was slightly enlarged and the pleurae and pericardium showed clear yellowish fluid. The second patient, the younger brother of patient one, had congenital lactic acidosis but no other symptoms until six months of age when he developed progressive muscle weakness. Treatment with dichloroacetate lowered the serum lactic acid level but did not affect his clinical condition. Cardiomyopathy was diagnosed at 13 months of age and he died of circulatory failure at 29 months. Both patients had mitochondrial myopathy with changes in skeletal muscle and the myocardium. Biochemical investigations of skeletal muscle mitochondria showed deficiencies in cytochrome c oxidase and NADH ferricyanide reductase. 
COMMENT. Patients with mitochondrial myopathies or cytopathies show marked heterogeneity in clinical manifestations and system involvement. Two major variants of mitochondrial myopathy and cytochrome c oxidase deficiency in infancy have been described. Most cases are rapidly progressive and fatal and are associated with renal dysfunction; occasionally the course is milder and reversible. In the present study, the heterogeneity in the mitochondrial cytochrome c oxidase activity provided clinical symptoms in proportion to the fraction of damaged mitochondria, thus explaining the different clinical course in the siblings.