Fourteen critically ill neonatal and paediatric intensive care patients with various primary diagnoses and signs of associated pulmonary hypertension received inhaled nitric oxide (NO), 20-80 ppm, after failure of conventional therapy to improve oxygenation. NO administration was found to be associated with a significant improvement in postductal arterial oxygen tension (pre-NO: 3.75 (SD 1.39) kPa; post-NO: 6.05 (SD 1.70) kPa; p = 0.004). In 10 patients, NO was found to increase arterial oxygen tension with more than 1 kPa. In 2 of these patients, ECMO treatment could be avoided due to the pronounced improvement in gas exchange seen after the initiation of NO administration. The remaining 4 patients failed to respond to NO administration. One patient developed methaemoglobinaemia (13.9%) which required treatment with methylthionine. Since we were unable to produce any beneficial effect of NO in the late phase of the pulmonary disease process, we believe that, in order to be successful, inhaled NO should be instituted when conventional treatment has failed and the administration of an iv vasodilator is usually considered.