The urea cycle converts ammonia into urea and defects of all the steps are well documented. Metabolic decompensation is associated with hyperammonemia, potentially leading to life threatening encephalopathy. Decompensation is often triggered by metabolic stress such as intercurrent illness, fasting, diarrhoea or vomiting, but an obvious precipitant is not always apparent. Patients presenting to A&E must be treated seriously, especially if they are vomiting or have impaired consciousness. Early intervention is essential to prevent cerebral oedema becoming established and irreversible. Treatment is aimed at reducing the production of ammonia so patients are treated with a low protein diet and medicines that promote the removal of nitrogen by alternative pathways.