We report an infant with neonatal nonketotic hyperglycinemia (NKH), diagnosed early and treated with dextromethorphan (DM) and sodium benzoate therapy from the 65th hour of life. Initially the patient responded to treatment showing clinical and electroencephalographic improvement: myoclonic jerks disappeared, muscular tone, reactivity to stimuli and spontaneous movements increased, assisted ventilation was no longer necessary and bottle feeding was initiated successfully; on EEG the suppression-burst pattern disappeared and the background activity was well-organized. At three months of age he developed flexor spasms and hypsarrhythmia. In spite of increasing doses of DM as high as 40 mg/kg/day and persistent therapy with sodium benzoate the child progressively deteriorated and died at the age of 5 months and 7 days. We stress that the adverse clinical course occurred in our patient even though DM and sodium benzoate therapy was started much earlier than in other reported cases. It is possible that prenatal brain damage and probable genetic variants (i.e. absent or minimal residual enzymatic activity and interindividual variations in DM metabolism) affect the response to therapy.