The cardio-pulmonary response to incremental and steady-state bicycle exercise was measured in a group of 22 patients with diffuse interstitial lung disease (DILD). In most patients the lung volumes were less than 80% of their predicted normal value. In 19 patients the uptake of carbon monoxide was less than 60% of the value predicted for the observed lung volume. Maximum working capacity (Wmax) was reduced in 20 patients. Eight patients stopped exercising before they achieved their predicted maximum heart rate. In all patients ventilation (VE) was increased in relation to oxygen consumption. The increase in VE resulted from an increased frequency of breathing. Exercise induced a decrease in arterial oxygen tension in 11 patients. When 60% oxygen was inspired during exercise the endurance time at 80% Wmax increased from 8.0 +/- 5.5 to 13.9 +/- 8.7 min. The increase in endurance time related to the extent of desaturation during exercise with air breathing. Oxygen breathing did not improve Wmax. We conclude that exercise testing in patients with DILD is useful for determining the severity of disease and provides additional information in relation to gas exchange.