We studied a child with a rapidly progressive neurologic disorder, with psychomotor retardation, hypotonia, seizures, and respiratory disturbances. Laboratory studies showed elevated levels of lactate and pyruvate in cerebrospiral fluid (CSF), without notable elevated levels in serum. In liver, muscle, leukocytes, and cultured fibroblasts we found no abnormality in pyruvate oxidation; biochemical studies of a brain biopsy showed an isolated deficiency of pyruvate dehydrogenase complex in brain tissue with the morphologic picture of progressive polio-dystrophy with hypomyelination.