The characteristics of dyspnoea in idiopathic pulmonary fibrosis (IPF) during a 6-min walk test are not clear. This study was designed to evaluate dyspnoea and desaturation during the 6-min walk test in IPF in comparison with that in chronic obstructive pulmonary disease (COPD), which is one of the most studied chronic lung diseases. The 41 consecutive patients with IPF included in this study were assessed by a 6-min walk test and concurrent measures of disease severity. Forty-one age-matched and resting PaO(2) value-matched COPD patients who had undertaken the test during the same period were selected as the control. Only O(2) saturation at the end of the test was an independent predictor of dyspnoea in IPF (r(2)=0.27, P=0.0005), whereas forced expiratory volume in 1s (FEV(1)) was the only predictor in COPD (r(2)=0.16, P=0.0096). Desaturation was significantly more severe in IPF (83.6+/-9.1% in IPF versus 88.0+/-5.9% in COPD, P<0.001). In contrast, dyspnoea assessed with the Borg scale was significantly more severe in COPD (3.6+/-2.1 in IPF versus 4.6+/-1.9 in COPD, P<0.05). O(2) saturation is an independent predictor of dyspnoea at the end of a 6-min walk test in IPF. In comparison with COPD, desaturation is more severe, although dyspnoea is milder.