To analyse the pattern of pulmonary gas exchange during maximal exercise (Emax) in asbestosis, we compared nine subjects with this disease (1 female/8 male), aged 54 +/- 11 yrs (mean +/- SD), to nine patients (1 female/8 male) with idiopathic pulmonary fibrosis (IPF) of a similar age, height, weight and smoking history, both at rest and during Emax. No differences were observed in dynamic and static lung volumes between the groups. However, patients with IPF had a lower DLCOsb and KCO (p less than 0.005 and 0.05, respectively). At rest, both groups showed mild arterial hypoxaemia (76 +/- 11, asbestosis, vs 77 +/- 11 mmHg, IPF), widened AaPO2 (32 +/- 14 vs 31 +/- 13 mmHg) and slight increases in VD/VT (47 +/- 12 vs 46 +/- 11%), respectively. During Emax, PaO2 fell to 51 +/- 7 mmHg in patients with IPF whereas those with asbestosis had PaO2 of 73 +/- 21 mmHg (p less than 0.05). Conversely, those with asbestosis were able to reduce VD/VT (from 47 +/- 12 to 39 +/- 10%, p = 0.01) as opposed to those with IPF (from 46 +/- 11 to 47 +/- 13%). Furthermore, DLCOsb and AaPO2 during Emax were highly correlated only in IPF (r: -0.84, p less than 0.01). Despite the finding that both diseases represent a diffuse pulmonary fibrosis with a similar degree of resting ventilatory impairment, the pattern of gas exchange during exercise is different in each. These differences may be related to the underlying morphology of each process, which probably includes more airway disease and less pulmonary vascular involvement and/or a different degree of interstitial fibrotic change in asbestosis.