Diffusion capacity for carbon monoxide of the lung (DLCO) before-and-after exercise test and 5 routine pulmonary function tests were conducted in 16 patients with diffuse pulmonary fibrosis (DPF), 19 patients with interstitial pneumonia (IP), 17 patients with COPD, and 22 normal subjects. The data showed: in normal subjects the DLCO after exercise increased significantly compared with before (P < 0.001). But in DPF group the DLCO before exercise was below the normal range, and it went down further after exercise, the decreasing rate being 17.95% (P < 0.001). The DLCO in IP and COPD groups did not significantly change before and after the exercise. Two cases of interstitial pneumonia, who had a reduced DLCO after exercise, were followed up for 0.5-1.0 year, and both patients developed into DPF by that time. The results suggest that the DLCO measure before and after exercise test may have important value for DPF diagnosis, and a reduced DLCO after exercise test in IP patients may warn the development from IP into DPF.