Ventilation was analysed during a maximal and progressive exercise in 12 patients suffering from interstitial lung disease (ILD) and in 15 healthy subjects. "Rapid shallowing breathing" pattern was observed in patients with ILD. Both normals and patients showed a linear relationship between VE, VT/TI (mean inspiratory flow) and VCO2. The relationship between P0.1, the occlusion pressure at 0.1 S, and VCO2 exponential in both groups and significantly steeper in patients with ILD. P0.1/VE and P0.1/(VT/TI) ratios, indexes of thoracopulmonary impedance, and P0.1/VCO2 ratio, an index of respiratory drive per unit of CO2 output, were all inversely correlated in patients with VC% (vital capacity) and CLstat (static compliance). These results strongly suggest that in patients suffering from ILD the higher respiratory drive observed during exercise could be attributed to increased afferent reflexes originating from the lung and/or chest wall.