A case which was suggested to be a new inborn error of tryptophan metabolism was described. The patient, a 9 year-old girl, was characterized by dwarfism, mental defect, photosensitivity and gait disturbance like cerebellar ataxia. These symptoms and signs were clinically similar to those in H disease but definitely different from it in laboratory findings. Laboratory studies revealed that the patient excreted an excessive amount of tryptophan without increase in indican or indoleacetic acid excretion. The tryptophan loading test exhibited that plasma level of tryptophan, following the loading, increased markedly and remained elevated longer accompanied with much increase in tryptophan and less increase in kynurenine in urine as compared with those in the controls. Basing upon the above-mentioned results, it was presumed that a biochemical lesion in the present disorder might lie in the conversion of tryptophan to kynurenine.