Experience with severe neonatal hyperammonaemia in the Newborn Intensive Care Unit of the Leuven University Hospital is reported and compared with the literature data. Among eleven patients seven presented with the transient neonatal hyperammonaemia-syndrome and four with a urea cycle defect. Prompt recognition of the transient neonatal hyperammonaemia syndrome is important because its prognosis is much better than that of the primary urea cycle defects presenting in the neonatal period.