The definition of neonatal hypoglycemia has been surveyed at the Department of Child Health, University of Newcastle upon Tyne by reference to 36 textbooks of pediatrics and answers to questionnaires from 178 pediatricians in charge of nurseries with 4 or more intensive care cots. In textbooks there was a wide variation in definition ranging from a glucose concentration of <1 mmol/1 to <2.5 mmol/1 with a modal value of <1.7 mmol/1 for term babies of appropriate weight, and <1.1 mmol/1 for babies who were preterm or small for gestational age. Among practicing pediatricians the definitions showed an even greater range from a glucose concentration of <1 mmol/1 to <4 mmol/1 with a modal value of <2.0 mmol/1 for term babies of appropriate weight and <1.1 mmol/1 for babies who were preterm or small for gestational age. The site of sampling — that is, whether capillary, venous, or arterial blood — was not included in the definition of hypoglycemia obtained from textbooks nor from pediatricians surveyed. There appeared to be no accepted definition of the lower limit of normality for circulating blood glucose concentrations. [1]