Major paediatric textbooks and the views of neonatologists in the United Kingdom were surveyed to establish a definition of neonatal hypoglycaemia. The definition ranged from a glucose concentration of less than 1 mmol/l to less than 4 mmol/l. Hypoglycaemia is recognised to cause neurological sequelae and yet there is no accepted definition of the lower limit of normality for circulating blood glucose concentrations.