Recent research has demonstrated that mutations of the hepatocyte nuclear factor 4‐alpha (HNF4A) gene are associated with neonatal hyperinsulinaemic hypoglycaemia. Mutations of this gene also cause one of the subtypes of monogenic diabetes, a form of diabetes formerly known as maturity‐onset diabetes of the young. This article describes a family discovered to have a novel frame‐shift mutation of the HNF4A gene in the setting of early‐onset maternal diabetes and severe neonatal hyperinsulinaemic hypoglycaemia. The implications of a diagnosis of HNF4A gene mutation for obstetric and paediatric practice are discussed.