The study goal was to define low thresholds of plasma glucose concentrations that constitute profound hypoglycemia. Population analysis was performed on research publications on neonatal hypoglycemia ascertained by a Medline search. Eligible patients had neurological sequelae associated directly and primarily with hypoglycemia (profound hypoglycemia). Of 89 infants, more than 95% had plasma glucose levels of less than 25 mg/dL that were first detected at more than 10 hours of age. A breakdown of study patients according to those with or without neurological sequelae, who were exposed to a similar degree of hypoglycemia, showed a combined incidence of neurological injury of 21%, with 95% confidence interval from 14% to 27%.