We evaluated 74 patients with pulmonary hypertension secondary to chronic obstructive pulmonary disease and compared pulmonary function tests as well as capillary blood gas values with pulmonary hemodynamic parameters. Thirty-four patients demonstrated pulmonary hypertension only at exercise, fourty patients had pulmonary hypertension at rest (mean pulmonary artery pressure greater than 20 mm Hg). There was a significant correlation between capillary oxygen tension and mean pulmonary artery pressure as well as pulmonary vascular resistance, both at rest and during exercise. The relation between capillary PCO2 and pulmonary hemodynamics were statistically less significant. The correlation between criteria of pulmonary obstruction and/or increased lung volumes and mean pulmonary artery pressure and pulmonary vascular resistance were not as significant. The 34 patients with pulmonary hypertension only at exercise differed significantly from the other groups of patients with pulmonary hypertension at rest as seen in all pulmonary function parameters. Furthermore there was also a difference in the capillary oxygen tension during exercise, but not in the capillary carbon dioxide tension. In conclusion, of all pulmonary function tests the most efficient criteria for pulmonary hypertension are reduced FEV1/VC and low oxygen tension at rest respectively a fall during exercise.