During a 4-year period (1984 to 1988), 50 children referred with manifestations of central nervous system or neuromuscular disease combined with hyperlactatemia were subjected to investigations that aimed to identify and characterize children with mitochondrial disorders. Biochemical and morphologic investigations of quadriceps muscle biopsy tissue were done, including oximetric and spectrophotometric analysis of the respiratory chain function, enzyme histochemistry, electron microscopy, and analysis of mitochondrial DNA. A diagnosis of mitochondrial disease was based on the presence of at least two of five criteria: (1) abnormal results of oximetry, (2) abnormal results of spectrophotometry, (3) enzyme histochemical evidence of cytochrome x oxidase deficiency, (4) deletions or point mutations of mitochondrial DNA, and (5) abundant ultrastructurally abnormal mitochondria. With the combined biochemical and morphologic investigation, 20 of the children were found to have mitochondrial disorders. In an additional 10 children a mitochondrial disorder was neither excluded nor verified. Mitochondrial disorders are thus an important cause of central nervous system and neuromuscular disease in children with hyperlactatemia.