Diazoxide is commonly used in the treatment of neonatal hyperinsulinism. We describe a one month-old infant who was treated with diazoxide for prolonged neonatal hyperinsulinism. Shortly after starting diazoxide, she was admitted to the hospital for tachypnea with hypoxemia, and was subsequently diagnosed with laryngomalacia and obstructive apnea. During hospitalization, her clinical course worsened due to the development of severe pulmonary hypertension, presumed due to diazoxide toxicity. Lung biopsy revealed a probable toxic vascular drug reaction. After discontinuing diazoxide, her clinical status improved and eventually returned to baseline. © Freund Publishing House Ltd., London.