Patent ductus arteriosus is one of the most common congenital abnormalities found in premature infants. Ibuprofen, a nonsteroidal drug that is commonly used as an antipyretic, analgesic and anti-inflammatory agent, is also used to induce closure of symptomatic patent ductus arteriosus in preterm infants. Recently, we gave L-lysine ibuprofen to a preterm infant with respiratory distress to induce closure of a patent ductus arteriosus, and the infant experienced pulmonary hypertension. Only 3 cases of pulmonary hypertension following early administration of an ibuprofen solution buffered with tromethamine have previously been reported. However, this severe side effect has never been observed in multicentre, randomized, double-blind controlled trials, nor in recent reviews or meta-analyses of L-lysine ibuprofen use.