The relationship between mean pulmonary artery pressure (PPA) and the regional distribution of perfusion, investigated with 133Xe, was studied in 67 patients with chronic respiratory diseases, including 57 patients with chronic obstructive pulmonary disease. The vertical gradient of perfusion, assessed by the ratio upper/lower perfusion (RQ), was considered as normal when RQ was less than or equal to 0.85. Patients were divided into three groups: no pulmonary hypertension, mild hypertension, moderate to severe hypertension. RQ was normal in the majority of patients, even in the group with PPA greater than 30 mm Hg. The average values of RQ did not significantly differ from one group to another. There was no correlation between PPA or pulmonary vascular resistance and RQ. These results suggest that the vertical gradient of perfusion is generally conserved and that the distribution of perfusion cannot provide any reliable prediction of pulmonary hypertension in chronic obstructive pulmonary disease.