Nonketotic hyperglycinemia is an inborn error of the glycine degradation pathway, presenting during the first days, even hours, of the newborn, and causing severe brain damage and often early death. The basic defect is in the glycine cleavage system, an intramitochondrial enzyme complex with four components; P, H, T and L proteins. Most cases with neonatal onset NKH have mutations in the gene coding for P protein(GLDC), some have mutations in the T protein gene (AMT). Only one patient with NKH has been described with a mutation in the H protein gene (GCSH). Glycine acts as neurotransmitter and its increased concentration due to deficient degradation has brain damaging effect. NKH does not affect the intrauterine development but after normal delivery the disease presents during the first days of life - 66 percent of cases were symptomatic before 48 hrs of life in one large series. Patients develop lethargy and profound hypotonia and refuse to feed. Wandering eye movements and intermittent ophthalmoplegia are frequent. As the encephalopathy progresses to coma, the infants develop frequent sequential myoclonic jerks, apneic episodes and hiccups. Most patients do not survive this stage without assisted ventilation. Even with assisted ventilation about 30 percent of patients die during the neonatal period. The surviving infants usually regain spontaneous respiration by 3 weeks of age, but are severely brain damaged until death at the age of few months to several years. The diagnosis of NKH is based on increased concentration of glycine in urine, plasma and cerebrospinal fluid. Cases in which the mutation of the family or the population is known can reliably and conveniently be diagnosed with gene test, which also is applicable for prenatal diagnosis.