PURPOSE: Leigh syndrome is a kind of mitochondrial disease with a pathological production of lactate and variable clinical manifestations. Recently biochemical assay of the mitochondrial enzyme activity and MRS are becoming a powerful tool for the definitive antemorterm diagnosis of this disease. The aim of this study was designed to determine the usefulness of biochemical enzyme assay and MRS in Leigh syndrome. METHODS: A clinical study of 8 cases of pediatric patients were diagnosed as Leigh syndrome on the basis of clinical manifestations and MRI features at the department of pediatrics, Ulsan university hospital, from July, 1992 to June, 1997. Biochemical enzyme assay of the cultured skin fibroblasts was consulted to hospital for sick children, Toronto, Canada. Cerebral lactates were detected on MRS were analysed in relation to the MRI findings and lactate levels in the CSF. RESULTS: 1) Age at diagnosis ranged from 7 months to 8.9 years; seven were boys and one was girl. 2) Main clinical symptoms were as follows in the order of frequency; seizure(75%), ophthalmoplegia(50%), hemiplegia(50%), hypotonia(50%), respiratory difficulty(50%), developmental delay(38%), consciousness change(38%), mental retardation( 25%). 3) Anatomical locations of the focal lesions detected on the initial MRI were as follows in the order of frequency; putamen(75%), caudate nuclei(63%), medulla oblongata (18,50%), substantia nigra(38%), thalamus(38%). 4) Biochemical enzyme assay revealed deficiency of NADH cytochrome c reductase (complex I) and deficiency of cytochrome c oxidase(complex N) respectively in 2 patients who had markedly elevated serum and CSF lactate levels and lactate/pyruvate ratio. 5) Cerebral lactates were detected by MRS from 6 lesions of 1 week-4 months old in 6 patients who had normal or mildly elevated serum and CSF lactate levels and lactate/pyruvate ratio. CONCLUSION: MRS detected the brain parenchymal lactate non-invasively from the acute or subacute lesions. Therefore, combined evaluation by MRS and biochemical enzyme assay will provide an important information for the diagnosis and estimation of disease activity in pediatric patients who are suspected to have Leigh syndrome.