Interstitial lung disease is characterized by interstitial inflammatory cell infiltration and fibrosis, a reduction in lung volumes, an increase in lung elastic recoil, and rapid shallow respirations. However, the alterations in lung volumes and elastic recoil as well as in breathing pattern are extremely variable, and the values are normal in a number of patients. In order to determine whether the effects of smoking could account for the variability in lung volumes and breathing pattern, we evaluated the elastic properties of the lung and the respiratory frequency at rest and during exercise in smokers and in nonsmokers with idiopathic pulmonary fibrosis (IPF) or with sarcoidosis. The volume-pressure curve in patients with IPF who smoked was positioned upwards and to the left when compared with that in nonsmokers. Conversely, the volume-pressure curve in patients with sarcoidosis who smoked was shifted downwards and to the right. In both conditions the respiratory rate while at rest and during exercise was greater in the group of patients who demonstrated the lower positioning of volume-pressure curves of the lungs (i.e., nonsmokers with IPF and smokers with sarcoidosis). We conclude that the impact of smoking may account, at least in part, for the variability in lung volume and volume-pressure characteristics of the lungs as well as the variability in respiratory rates in patients with interstitial lung disease.