Objective: To determine whether the devastating outcome of neonatal-onset glycine encephalopathy (NKH) could be improved by instituting treatment immediately at birth rather than after symptoms are already well established. Methods: A newborn with NKH diagnosed prenatally following the neonatal death of a previous affected sibling was treated from birth with oral sodium benzoate (250mg/kg/day) and the NMDA receptor antagonist ketamine (15mg/kg/day) immediately after sampling cord blood and cerebrospinal fluid (CSF) for glycine determination. Glycine cleavage system (CGS) activity was determined in placental tissue. Mutation analysis was performed by sequencing all GLDC, GCSH and AMT exons. Results: CSF glycine (99 μmol/L, reference 3.8-8.0) was already markedly elevated at birth. GCS activity in placental tissue was severely reduced (2.6% of controls). A novel homozygous GLDC C.482A→G(Y161C) missense mutation was identified. Neonatal hypotonia and apnea did not occur but the long-term outcome was poor, with intractable seizures and severe psychomotor retardation. This contrasts with the favorable outcome with early treatment in variant NKH with mild GCS deficiency (Ann Neuol 2004;56:139-143). Interpretation: Prospective treatment with this regimen can favorably modify the early neonatal course of severe NKH but does not prevent the poor long-term outcome, suggesting glycine-induced prenatal injury and/or ongoing postnatal damage.