A cases of adult respiratory distress syndrome, ARDS, was treated with nitric oxide (NO) inhalation for 10 days. The optimal dose of inhaled NO was determined by the dose-response relationship with NO concentration ranging from 5 to 160 ppm. 5 ppm of NO inhalation considerably increased PaO2 and decreased mean pulmonary arterial pressure (mPAP). Further increase in concentration of inhaled NO did not result in remarkable change in PaO2, but decreased mPAP. NO inhalation therapy was begun with 20 ppm of NO and 0.95 of FIO2, and on the 6th day NO concentration was decreased to 5 ppm with 0.7 of FIO2. In this case of ARDS, FIO2 could be sufficiently decreased and PaO2 maintained at more than 60 Torr with 5 ppm NO inhalation.