The management of congenital hyperinsulinism (CHI) has improved.To examine the treatment and long-term outcome of Finnish patients with persistent and transient CHI (P-CHI and T-CHI).A population-based retrospective study of CHI patient treated in 1972-2015.106 patients with P-CHI and 132 patients with T-CHI (altogether, 42 diagnosed before and 196 after year 2000) with a median follow-up times of 12.5 and 6.2 years, respectively.Recovery, diabetes, pancreatic exocrine dysfunction, neurodevelopment.The overall incidence of CHI (n = 238) was 1:11 300 live births (1972-2015). In 2000-2015, the incidence of P-CHI (n = 69) was 1:13 500 and of T-CHI (n = 127) 1:7400 live births. In the 21 st century P-CHI group, hyperinsulinemic medication was initiated and normoglycemia achieved faster compared to earlier. Of the 74 medically treated P-CHI patients, 68% had discontinued medication. Thirteen (12%) P-CHI patients had partial pancreatic resection and 19 (18%) underwent near-total pancreatectomy. Of these, 0% and 84% developed diabetes and 23% and 58% had clinical pancreatic exocrine dysfunction, respectively. Mild neurological difficulties (21% vs 16%, respectively) and intellectual disability (9% vs 5%, respectively) were as common in the P-CHI and T-CHI groups. However, the 21 st century P-CHI patients had significantly more often normal neurodevelopment and significantly more infrequently diabetes and pancreatic exocrine dysfunction compared to those diagnosed earlier.Our results demonstrated improved treatment and long-term outcome in the 21 st century P-CHI patients compared to earlier.