Background: It is known that pulmonary rehabilitation improves dyspnea and exercise tolerance in patient with chronic lung disease, although it does not improve pulmonary function. But there is a controversy whether this improvement after pulmonary rehabilitation is due to increased aerobic exercise capacity. We performed this study to evaluate the effect of pulmonary rehabilitation for 6 weeks on the pulmonary function, gas exchange, exercise tolerance and aerobic exercise capacity in patients with chronic lung disease. Methods: Pulmonary rehabilitations including education, muscle strengthening exercise and symptom-limited aerobic exercise for six weeks, were performed in fourteen patients with chronic lung disease (COPD 11, bronchiectasis 1, IPF 1, sarcoidosis 1 ; mean age 57 +/-4 years ; male 12, female 2). Pre- and post-rehabilitation pulmonary function and exercise capacity were compared. Results: 1) Before the rehabilitation, FVC, FEV1 and FEF25-75% of the patients were 71.5+/-6.4%, 40.6+/- 3.4% and 19.3+/-3.8% of predicted value respectively. TLC, FRC and RV were 130.3+/-9.3%, 157.3 +/-13.2% and 211.1 +/-23.9% predicted respectively. Diffusing capacity and MW were 59.1 +/-1.1% and 48.6 +/-6.2%. These pulmonary functions did not change after pulmonary rehabilitation. 2) In the incremental exercise test using bicycle ergometer, maximum work rate (57.7 +/-4.9 watts vs. 64.8 +/-6.0 watts, P=0.036), maximum oxygen consumption (0.81 +/-0.07 L/min vs. 0.96 +/-0.08 L/min, P=0.004) and anaerobic threshold (0.60 +/-0.06 L/min vs. 0.76 +/-0.06 L/min, P=0.009) were significantly increased after pulmonary rehabilitation. There was no improvement in gas exchange after rehabilitation. 3) Exercise endurances of upper (4.5 +/-0.7 joule vs. 14.8+/-2.4 joule, P< 0.001) and lower extremity (25.4+/-5.7 joule vs. 42.6+/-7.7 joule, P<0.001), and 6 minute walking distance (392+/-35 meter vs. 459+/-33 meter, P<0.001) were significantly increased after rehabilitation. Maximum inspiratory pressure was also increased after rehabilitation (68.5 +/-5.4 cmH2O vs. 80.4 +/-6.4 cmH20, P<0.001). Conclusion: The pulmonary rehabilitation for 6 weeks can improve exercise performance in patients with chronic lung disease.