Clinical observations and results of investigations of pyruvic acid metabolism are reported in 4 children in whom subacute necrotizing encephalomyelopathy of Leigh was diagnosed intravitally. Attention is called to the similarity of the clinical manifestations with its onset in the first year of life, deficient body weight and growth, progressing neurological disturbances (weakening of muscle power, tremor, ataxia, nystagmus), course with periods of exacerbations, tachypnoea, skin changes (hirsutism, telangiectasia, perspiration), death at the age of 2-3 years. The biochemical changes in all children included raised serum levels of lactic acid, pyruvic acid and alanine, and acid-base equilibrium disturbances with metabolic acidosis (relatively balanced respiratory alkalosis). The results of the test of intravenous loading with glucose and alanine carried out in all children indicated indirectly reduced activity of pyruvate carboxylase. In one child histological examination of the brain carried out postmortem confirmed the diagnosis of Leigh's disease.