Congenital and adult-onset hyperinsulinism (CHI) must be taken under consideration in the differential diagnosis of hypoglycaemia symptoms with endogenous hyperinsulinism, especially in cases in which there was failure to find an insulinoma. Histological examination is necessary for a definitive diagnosis. CHI is a disorder with three histopathological variants: focal CHI, diffuse CHI, and atypical CHI. These variants are clinically indistinguishable. According to published statistics, 0.5 to 5% of nesidioblastosis cases occur in adults. Clinical manifestation ranges from mildly symptomatic up to life-threatening hypoglycaemia. Early diagnosis and treatment are important in young and very young patients because early treatment accounts for favourable mental outcomes.