Forty patients with chronic bronchitis and emphysema qualified for domiciliary oxygen treatment according to widely accepted criteria were assessed. Only two patients had normal pulmonary artery pressure. Pulmonary arterial pressure correlated well with oxygen arterial partial pressure. It seems that pulmonary hemodynamic studies are not necessary in patient qualification for domiciliary oxygen therapy. Short oxygen therapy (30 min.) produced only a small decrease of pulmonary artery pressure from 31 +/- 10 to 29 +/- 8 mm Hg. Only in 7 patients was the fall greater than 5 mm Hg. Fall of pressure in pulmonary artery produced by oxygen correlated only with initial pulmonary artery pressure and rose proportionally with increase of pressure.