Congenital Hyperinsulinism (CHI) is a major cause of neonatal hypoglycemia characterised by non-ketotic hypoglycemia. We describe the occurrence and higher prevalence of ketotic hypoglycemia (KH) in 5 children with transient CHI. Four children had required diazoxide to control the persistent hypoglycemia that was discontinued at a mean age of 11.25 (+5.25) months. KH developed after an average time period of 6.7 months following the resolution of CHI. Children with transient CHI may be at risk of subsequently developing KH at a variable age period.