Nonketotic hyperglycinemia (NKH) is a rare autosomal recessive disorder resulting from a biochemical defect in the glycine cleavage enzyme system. This consists of four protein complexes (P, T, H, L), which are encoded by GLDC, AMT, GCSH, and GCSL, respectively. 1 In classical neonatal form, the patients' symptoms include lethargy, hypotonia, apnea, hiccups, and intractable seizures resulting in severe psychomotor disability. 2 Glycine functions as an inhibitory neurotransmitter in the brainstem and spinal cord, but in an excitatory manner in the cerebral cortex, the result of an agonist effect on N-methyl-D-aspartate receptors (NMDA-R). Current treatment consists of reducing glycine levels with sodium benzoate (250-550 mg/kg/day) and blocking the effect of excess glycine on NMDA-R with dextromethorphan (5-22 mg/kg/day). A high dose of benzoate (550-750 mg/kg/ day) would be attempted for severe NKH patients. Average survival time of neonatal NKH who survived newborn period is reported to be 3 years 11 months (2.3 months to 15 years). 3 The patient was a Japanese male, aged 17 years, who had been followed up since he was a neonate. He was born at 40 weeks gestation, with a normal pregnancy, and delivery was through cesarean section without complications. The patient developed poor sucking, hypotonia, convulsions, and apnea from 2 days of age. General laboratory tests were not remarkable. He was diagnosed with NKH via elevated glycine levels in the cerebrospinal fluid (CSF) at 323.7 lmol/L (normal range: <25 lmol/L) and in the plasma at 1,729.6 lmol/L (normal range: 125-343 lmol/L), with a CSF/plasma glycine ratio of 0.19 (diagnostic values: >0.08). A genetic analysis was performed after obtaining written informed consent from the patient's parents. The patient demonstrated compound heterozygosity for a deletion of exons 3-8 and a novel c.2574T >G (p.Y858X) mutation in the GLDC gene, as well as decreased 13 C-CO 2 exhalation in a 13 C-glycine breath test. The patient was treated with high-dose benzoate (600 mg/kg)