Glycogen storage disease type 1 (GSD1) is a disorder of glycogen breakdown and gluconeogenesis. Adult patients with GSD1 may become hypoglycaemic if they fast for longer than about 3-4 hours: sometimes it is after a much shorter time. During illness, patients have an even greater tendency to become hypoglycaemic. The treatment aims to maintain normoglycaemia all the time either with frequent oral drinks, a continuous infusion via a naso-gastric tube or gastrostomy or an intravenous infusion of glucose. This management should be started as soon as patients become unwell. The early signs of decompensation may be subtle and some patients have loss of hypoglycaemic awareness and may remain asymptomatic despite marked hypoglycaemia and metabolic acidosis. Other may present with a hypoglycaemic convulsion.