OBJECTIVE Pulmonary capillary blood volume (Qc), a component of diffusing capacity of the lung for carbon monoxide (Dlco), is increased in postcapillary pulmonary hypertension due to valve disease, but is decreased in primitive and thromboembolic pulmonary hypertension. This study was performed to evaluate which way pulmonary Qc is affected in patients with chronic infiltrative lung disease according to the value of systolic pulmonary artery pressure (SPAP). METHODS Twenty-four patients who were nonsmokers and had chronic infiltrative lung disease secondary to connective tissue disease (12 patients), asbestosis (1 patient), sarcoidosis (5 patients), or of unknown origin (6 patients), and 8 control subjects underwent pulmonary function tests and Doppler echocardiography. RESULTS Total lung capacity, alveolar-arterial oxygen pressure difference, Dlco, and conductance of the alveolar-capillary membrane (Dm) did not differ between patients with low SPAP (LPAP) [ie, < 30 mm Hg] or high SPAP (HPAP). Patients with LPAP, but not HPAP, experienced significant decreases in pulmonary Qc, whatever the cause of the disease. There was a strong positive correlation between SPAP and Qc scaled by Dm to account for infiltrative disease severity (r = 0.68; p < 0.001). CONCLUSIONS We thus conclude that pulmonary Qc is not decreased as expected in patients with chronic infiltrative lung disease and high pulmonary artery pressure. A high Qc/Dm ratio should encourage the physician to look for HPAP compatible with pulmonary hypertension, whatever the etiology of lung infiltrative disease.