To investigate pulmonary hemodynamics in patients with chronic respiratory failure, we performed right heart catheterization and also examined the effects of oxygen inhalation. Thirty-three patients were studied. Their mean age was 65 +/- 10 years, and FEV1.0% and PaO2 on room air were 47 +/- 16% and 63 +/- 14 mmHg, respectively. mPAP was 27 +/- 10 mmHg, and 26 (78%) had pulmonary hypertension. Although PVR was abnormally high, CI and PCWP were within normal limits. PaO2 was significant Significantly correlated with mPAP and PVR. Although oxygen inhalation (28%) for 30 minutes significantly changed PaO2, PaCO2, and PvO2, it did not significantly affect mPAP, PVR, CI, or PCWP. These results suggest that short-term oxygen inhalation does not affect pulmonary hemodynamics in patients with chronic respiratory failure. Radionuclid ventriculography was also done to evaluate cardiac function in 16 patients before and after home oxygen therapy (HOT). Both right ventricular ejection fraction (RVEP) and left ventricular ejection fraction (LVEF) were significantly increased by HOT, and the significant correlation between RVEF and LVEF that had been observed before HOT disappeared. These results suggest cardiac function improved and that the interdependence between the right and left ventricle was eliminated by HOT.