The growing number of patients treated with long-term oxygen therapy (LTOT) poses the question which physiologic variables could predict the patients who may benefit the most from this cumbersome and expensive treatment. We wanted to verify if the acute effect of oxygen on pulmonary arterial pressure (PAP) is related to survival on LTOT as was suggested recently in the literature. We studied 46 chronic obstructive pulmonary disease (COPD) patients qualified for LTOT. The acute effects of O2 on pulmonary hemodynamics were assessed by pressure and flow measurements before and after 30 min of O2 breathing via 28% Ventimask. Thirty-nine patients reacted with a fall of the mean PAP of less than 0.7 kPa 5 (mm Hg). These were termed nonresponders (NR). In seven patients, mean PAP fell greater than or equal to 0.7 kPa 5 (mm Hg). They were called responders (R). After the initial investigations, patients were followed up on LTOT for 2 yr or until death. During the first year, four patients died; three from the NR and one from the R group. After 2 yr of LTOT, 15 patients died (12 from NR and 3 from R groups). The 2-yr survival rate was 69% in NR and 57% in R groups, respectively. We conclude that survival on LTOT is not related to the acute effect of oxygen on the PAP in COPD patients investigated in the steady-state period of the disease.