Arterial oxygen tension (PaO2) was found to decrease linealy during incremental exercise in patients with chronic lung diseases. The slopes of PaO2 during increasing workload (delta PaO2/delta VO2) in patients with COPD were closely and negatively correlated with diffusing capacity of the lung (DLco). In patients with IPF, the values of delta PaO2/delta VO2 were much higher, and were more strongly correlated with DLco than those in patients with COPD. Therefore, exercise-induced hypoxemia (EIH) was found to be related to three factors: PaO2 at rest, PaO2 slope predicted from %DLco, and exercise workload. The most important factor in EIH in patients with COPD was the decrease in PvO2 during exercise, which was caused by right heart failure and venous admixture. In patients with IPF, EIH was caused by abnormal O2 diffusion during exercise, decreased PvO2, and venous admixture. In conclusion, the factors contributing to EIH depend on the workload, so exercise testing is needed to decide on exercise therapy and O2 therapy in patients with chronic lung diseases.