We examined the effects of the stopping of oxygen inhalation on pulmonary hemodynamics in 19 patients with stable chronic cor pulmonary with 1 year of Home Oxygen Therapy (HOT). All patients were classified into two groups in which hemodynamic examination was performed under oxygen inhalation (Group A; 10) or under air breathing from 4 hours before examination (Group B; 9). The hemodynamic examination was repeated after 2 hours of stopping oxygen inhalation in Group A, and was repeated 2 hours after restarting oxygen in Group B. The mean pulmonary arterial pressure (mPAP) decreased significantly from 25.2 +/- 7.2 Torr before HOT to 20.4 +/- 3.2 Torr after 1 year of HOT, and then increased to 27.6 +/- 5.6 Torr after stopping oxygen. In contrast, mPAP in Group B did not change: 32.9 +/- 9.4 Torr before HOT, 29.4 +/- 8.2 Torr during stopping of oxygen and 27.6 +/- 5.6 Torr 2 hours after restarting oxygen after 1 year of HOT. These findings suggest that stopping oxygen inhalation in patients with long-term HOT can markedly affect pulmonary hemodynamics as well as the prognosis.