A study was made of pulmonary artery pressure in 16 chronic bronchitics with severe respiratory failure, who were treated with continuous oxygen therapy at home. At the beginning of the study these patients presented with a pure and severe obstructive syndrome with marked hypoxaemia at rest (mean PaO2 = 6.8 kPa), had a suitable interval from a recent exacerbation, in a stable clinical and functional respiratory state. The duration of longterm oxygen therapy was controlled, as the efficacy of the blood gases (PaO2 of O2 greater than or equal to 8 kPa). The pulmonary artery pressure was measured using a microcatheter, at the beginning, and for most of the 18 months breathing ambient air in a stable state. At the end of this time a significant diminution of the FEV1 (VEMS) (a mean of 100 ml, p less than 0.01). This was evidence of the deterioration in the ventilatory state which contrasted with the improvement in the diminution of pulmonary artery hypertension (HTAP) from 3.43 to 2.97 kPa (p less than 0.05) without the deterioration in the other values. The good result above in chronic bronchitics contrasts with a stable or deteriorating pulmonary arterial pressure in a group of emphysematous (n = 6) and bronchiectatic subjects (n = 4) with somewhat similar functional characteristics but in whom the effect was too small to allow for any statistical comparison. Our results are compared with those in the literature much of which applies to only a few patients and for the great part revealed only haemodynamic stability of oxygen therapy. Possible reasons for these discordant findings are discussed.