The volume and time components of the breathing cycle together with the mouth occlusion pressure P0.1) were measured in 12 patients with chronic diffuse lung fibrosis (DLF) breathing air and in nine normal subjects matched for age and sex. In addition, static lung elastance was measured in all patients. It was found that in DLF the frequency of breathing (f) and P0.1 were positively correlated with lung elastance, whereas tidal volume (VT) decreased significantly with increasing elastance. Minute ventilation and mean inspiratory flow were independent of lung elastance. The VT and f of DLF reported in this paper and those from other published reports were compared with the VT and f of elastically loaded normal subjects. At equivalent levels of elastance, both DLF and loaded normal subjects show qualitatively similar changes. This data suggests that in DLF patients the inspiratory neuromuscular drive is increased as a function of their elastic load, the increase being sufficient to maintain alveolar ventilation within normal limits.