Amino acids analysis were made on serum and cerebrospinal fluid samples of a Japanese 5-month-old infant suffering from irritability and mental retardation noticed at 2 months of age. Excessive amounts of branched-chain amino acids and of keto acids were detected in those samples and the large quantity of keto acids was found in urine with a qualitative 2,4-dinitro-phenyl-hydrazin test and with quantitative estimation. When thiamine hydrochloride (100 mg/day) was administered orally for 7 days to the patient fed with the cow's milk formula containing 2.1 gm/dl milk protein, there was no improvement of the branched-chain amino acidemia. Urinary keto acids, however, showed a marked decrease 7 days after the administration of thiamine hydrochloride. An overnight fast for 13 h resulted in normoglycemia. There was found no difference of blood L-lencine level between both parents and normal infants to whom L-leucine was loaded. The relation between decarboxylase activity for keto acids of branched-chain amino acids and thiamine hydrochloride was studied clinically, in the present communication.