Background: Pulmonary emphysema, with or without chronic bronchitis, has emerged as the most common indication for successful single lung transplantation. Although gas exchange can be expected to improve after successful transplantation, such changes do not adequately explain the improvement in dyspnea experienced by these patients and resulting in improved quality of life. Methods: We prospectively studied the respiratory mechanics of 14 single lung transplantation recipients with pulmonary emphysema, of whom 10 have been followed up beyond 1 year. The mean age of the group was 48.8 years (range, 42 to 60 years) for the seven men and seven women. Average donor-predicted total lung capacity was 0.6 L (+ 0.2 [standard error]) greater than recipient-predicted total lung capacity. Comparison of pulmonary resistance, dynamic lung compliance, and static lung compliance were taken to examine the possible role of reduced airways resistance and of improved elastic recoil in the reduced work of breathing. Results were analyzed by means of a one-tailed paired Student t test and linear regression analysis (both stepwise and multivariate); results are tabulated by mean (± 1 [standard error]). Results: Between preoperative measurements and 12 months postoperatively, maximum lung elastic recoil increased from 8.1 (± 0.7) to 11.3 (± 1.0) cm H2O, p < 0.001; pulmonary resistance fell from 8.3 (± 0.8) to 5.4 (± 0.7) cm H2O sec/L, p < 0.01. Dynamic lung compliance fell from 0.23 (± 0.04) to 0.12 (± 0.02) L/cm H2O,p < 0.02, and static lung compliance fell from 0.66 (± 0.13) to 0.22 (± 0.05) L/cm H2O,p < 0.001. Conclusions: The decline in lung compliance after single lung transplantation reflects the importance of improvement in elastic recoil and reduced chest wall distention, improving the work of breathing. The 67% decline in static lung compliance (300% increase in elastic recoil) is probably the single most important mechanical factor leading to reduced dyspnea after single lung transplantation for emphysema.