Concentrations of phosphorus metabolites in brains of 4 infants with the West syndrome were measured by 31P MRS before and during ACTH treatment. Although intracellular pH, phosphocreatine/ATP ratio and phosphocreatine/inorganic phosphate ratio in the patients were similar to those in the controls and did not change after the ACTH treatment, phosphomonoesters/phosphodiesters ratio in the patients always decreased with ACTH treatment. These results indicate that ACTH may accelerate the maturation of phospholipids metabolism in infant brains, and the observed change may relate to the efficacy of ACTH treatment against the West syndrome.