We examined pyruvate turnover using 1-[11C]pyruvate in the brain and epicranial muscle of 6 patients with mitochondrial encephalomyopathy (MEM), diagnosed by muscle biopsy and mitochondrial enzyme assay. The radioactivity was measured by positron emission tomography (PET). The time-activity curve for 11C in both brain and muscle generated after i.v. injection of 1-[11C]pyruvate consisted of 2 components in normal subjects and patients, i.e. a fast and a slow component which were assumed to represent the aerobic (mitochondrial) and anaerobic (glycolytic) metabolism of pyruvate, respectively. In the brain and muscle of patients, the aerobic component was smaller and the anaerobic larger than in normals. The extent of this abnormality seemed to reflect the severity of the disease. The same slight abnormality for [11C]pyruvate turnover was also observed in the brain of MEM patients who were without cerebral symptoms. Cerebral blood flow (CBF) and cerebral oxygen consumption (CMRO2) of most patients were lower than those of normals, and the oxygen extraction fraction (OEF) was decreased in many patients.