A case of idiopathic hyperglycinemia was reported. This was the third case of this particular disorder next to Childs et al.'s and Nyhan et al.'s cases. A striking elevation of glycine in urine and serum from the patient was demonstrated by the use of Automatic Amino Acid Analyzer. (This work was supported in part by a Grant in Aid for Developmental Scientific Research of Ministry of Education. Prof. Ts. Arakawa)