Neonatal hyperinsulinism, whether permanent or transient, results in prolonged hypoglycemia, which increases the risk of hypoglycemic brain injury. Therefore, prompt diagnosis and management of hyperinsulinemic hypoglycemia is important. Drawing a “critical sample” at the time of hypoglycemia is useful for diagnosis. Genetic testing for defective insulin-regulating genes in pancreatic beta-cells might also be helpful in cases of prolonged hypoglycemia. High-calorie feeding or glucose infusion is necessary to maintain normoglycemia. Diazoxide is the treatment of choice for hyperinsulinism and should be continued until the hypoglycemia resolves. We describe a case of transient neonatal hyperinsulinemia hypoglycemia in a small-for-gestational-age preterm infant who underwent diazoxide treatment and achieved a favorable outcome.