An amino acid formula produced in Japan is not supplemented with biotin since biotin is not permitted as a food additive. Biotin deficiency developed in an 11-month-old Japanese infant who had been diagnosed as a neonate with cow milk and soy bean allergy and fed with an amino acid formula and hypoallergenic rice processed by protease. Serum levels of zinc, essential fatty acids and biotinidase were within the normal range while that of biotin was below the normal range. Urinary 3-hydroxy-isovalerate and slightly elevated levels of plasma branched-chain amino acids disappeared 1 week after oral supplementation with 1 mg day-1 of biotin as did the symptoms of orificial skin lesions, lethargy, hypotonia and alopecia later. In summary, to prevent biotin deficiency, biotin should be added to the Japanese amino acid formula.