Four siblings with Leigh's syndrome are described. The diagnosis was confirmed by pathological examination in one case. Chemical and biochemical investigations of serum and urine revealed no abnormalities of pyruvate metabolism, but all patients had marked elevations of CSF pyruvate and lactate concentrations. In three of the siblings, [1-14C]pyruvate oxidation rates were normal in fibroblasts and leucocytes. In one patients, extensive biochemical and histochemical studies of liver and muscle tissue revealed no mitochondrial dysfunction. A defect of oxidative metabolism restricted to brain seems probable.