We wanted to verify if the acute effect of oxygen on pulmonary arterial pressure (PPA) is related to survival on LTOT as was suggested recently in the literature. We studied 46 COPD patients qualified for LTOT. The acute effects of O2 on pulmonary haemodynamics were assessed by pressure and flow measurements before and after 30 mins of O2 breathing via 28% Ventimask. Thirty nine patients reacted with a fall of the mean PPA of less than 5 mm Hg. These were termed nonresponders (NR). In seven patients mean PPA fell > 5 mm Hg. They were called responders (R). After the initial investigations patients were followed up on LTOT for 2 years or until death. During two years of LTOT 15 patients died (12 from NR and 3 from R groups). The first two years 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 pulmonary arterial pressure in COPD patients investigated in the steady-state period of the disease.