This report concerns 26 cases of hyperinsulinism in neonates and infants. Clinically, the syndrome beginning in the neonatal period seems to differ by several aspects from the disease with a later onset: diagnostic difficulties, treatment modalities and long term prognosis. Insulinemia determined when the patient presents with hypoglycemia is often (75 to 80% of cases) normal. The diagnosis of hyperinsulinism has to be based on the clinical characteristics of hypoglycemia (timing of the episodes, response to glucagon) and on specific biological data (C-peptide, plasma ketone bodies and branched-chain amino acids at the time of hypoglycemia induced by fasting). The pathogenesis of hyperinsulinism is still poorly understood and has several etiologies.