Evaluation of the gas exchange responses to steady state treadmill exercise of 176 patients with chronic disorders of the lower respiratory tract demonstrated that alveolar disease significantly affects O2 transfer with little effect on CO2 transfer. At exercise levels requiring oxygen delivery 2- to 3-fold above resting levels, patients with interstitial lung disease [idiopathic pulmonary fibrosis (IPF), sarcoidosis, histiocytosis-X (HX)] and "pure" destructive lung disease [alpha l-antitrypsin (alpha IAT) deficiency] demonstrated no ventilation or cardiac limitation, but all exhibited marked reductions in O2 transfer. Strikingly, although resting values of PaO2 were normal or mildly reduced, the average estimated PaO2 at a VO2 of 1 L/min was 47 mmHg in IPF, 65 mmHg in sarcoid, 54 mmHg in HX, and 41 mmHg in alpha IAT deficiency. Comparison of resting and exercise parameters revealed that: (1) exercise studies can uncover alveolar dysfunction in the presence of normal resting parameters, and (2) resting parameters cannot predict the magnitude of O2 related abnormalities induced by exercise. These observations suggest that exercise testing is a useful clinical tool in detecting alveolar disease and gauging the magnitude of abnormalities of O2 transfer.