Congenital hyperinsulinism is considered to be the most frequent cause of persistent recurrent hypoglycaemia in infants. The clinical presentation and response to pharmacological treatment may vary significantly depending on the underlying pathology. We report a case of a female infant with mild but early onset of recurrent hypoglycaemia. Metabolic workup revealed hyperinsulinism combined with mild hyperammonaemia as well as elevation of α-ketoglutarate in urine. Genetic testing demonstrated a de novo mutation in exon 7 of the glutamate dehydrogenase gene on chromosome 10. Episodes of hypoglycaemia responded to treatment with diazoxide. The differential diagnosis, pathophysiology and treatment of congenital hyperinsulinism is discussed.