The guidelines of the Global Initiative for Chronic Obstructive Lung Disease (GOLD) do not recommend the measurement of pulmonary artery pressure in patients with chronic obstructive pulmonary disease (COPD). This is on the basis that the mean pulmonary artery pressure (mPAP) does not provide more clinical information than measurement of the oxygen tension in arterial blood (PaO2). The mPAP correlates well with PaO2 in emphysema patients with severe hypoxemia (PaO2 < or = 7.3 kPa (55 mmHg)). However, the occurrence and significance of mPAP is unclear in patients without severe hypoxemia (PaO2 > 7.3 kPa (55 mmHg)). In order to evaluate the usefulness of measurement of mPAP in emphysema patients without severe hypoxemia, we performed right heart catheterization and investigated the pulmonary hemodynamics of 53 patients without severe hypoxemia. In addition, we identified long-term prognostic factors with a mean follow-up term of 77 months after right heart catheterization. Seventeen of 27 patients with mild-to-moderate hypoxemia exhibited pulmonary hypertension (mPAP > or = 2.7 kPa (20 mmHg)) and the classification according to severity in GOLD exhibited a greater correlation to mPAP than PaO2. Moreover, only mPAP was found to be a significant prognostic factor according to multivariate proportional hazards analysis (P = 0.01). We conclude that mPAP is more informative about the severity of emphysema than PaO2 in patients with mild-to-moderate hypoxemia.