Diazoxide is the main therapeutic agent for persistent hyperinsulinemic hypoglycemia. Generally, it is tolerated well, but rarely it can cause severe life-threatening complications. We report a neonate who was treated with diazoxide for hyperinsulinemic hypoglycemia. On the 6th day of the treatment we observed sepsis-mimicking symptoms, mild pulmonary hypertension, and re-opening of the ductus arteriosus. All these findings resolved dramatically shortly after discontinuation of treatment. To our knowledge, this is the first reported case of re-opening of the ductus arteriosus due to diazoxide toxicity.