The purpose of the study was differential diagnosis of lactic acidosis in 44 children aged from 2 weeks to 4 years. In all of them the lactate level in repeated determinations exceeded 27 mg/100 ml. From the point of view of clinical manifestations the children were divided into three groups: 26 with hepatomegaly and hypoglycaemia (I), 6 with ataxia and retardation of somatic development (II), 12 with mental retardation and muscular hypotonia (III). Together with basic biochemical studies other tests were done, if necessary, including glucose and alanine loading, lactate determination in cerebrospinal fluid, analysis of urinary organic acids by the GC-MS method, morphological examinations of muscle biopsy material, enzymatic determinations in liver biopsy material. In group I glycogenosis was suspected and its type was finally established after biochemical and enzymatic tests (types I, Ib, III, VI, VIa, XI). In one case fructose-1,6-diphosphatase deficiency was suspected. In group II the clinical manifestations resembled Leigh's syndrome. The tests demonstrated an inhibition of glucose formation from alanine, and lactate level in the cerebrospinal fluid was evidently raised above that in the serum. Gasometric index showed the presence of respiratory alkalosis with metabolic compensation rather than primary lactate acidosis. In group III, with considerable clinical variety of signs, in only nine out of 12 children the cause of lactate acidosis could have been established (pathological changes of mitochondria in 4 cases, secondary increase of lactate without pathogenetic importance in 4, and 3-hydroxy-3-methylglutaric acidosis in 1 case. In conclusion it is thought that this combination of diagnostic methods is useful in differential diagnosis of congenital lactate acidosis in children.