65 patients with interstitial lung diseases were subdivided into two groups: idiopathic pulmonary fibrosis (IPF) and other interstitial pulmonary diseases (OIP) according to histopathological, immunological and follow-up findings. Arterial blood gases and pulmonary hemodynamics at rest and during steady-state exercise were compared in these two groups (IPF= 31 patients, OIP= 34 patients). The resting PaO₂ was significantly lower in the IPF group (p < 0.001l) and regularly worsened during exercise in this group (from 69.6 ± 11.6 to 56.0 ± 9.4 mm Hg, p < 0.001 whereas it did not vary significantly in the OIP group (from 79.2 ± 13.1 to 75.3 ± 14.1 mm Hg, NS). Pulmonary artery mean pressure (Ppa) was higher in the IPF group (p < O.OOl) as were all indices of pulmonary vascular resistance (PVR). During exercise, P_PA markedly increased in the IPF group (from 21.7 ± 7.8 to 45.3 ± 16.2 mm Hg) but modestly in the OIP group (from 15.5 ± 3.7 to 28.3 ± 9.8 mm Hg). The best relationships were found between P_pa, PVR and PaO₂, which suggests that pulmonary arterial hypertension in these diseases is not only due to anatomical factors but also to alveolar and arterial hypoxia. Arterial blood gases and pulmonary hemodynamics at rest and during exercise may be of interest for the discrimination of’true’ interstitial fibrosis (IPF) from other interstitial lung diseases (OIP).