Cardiopulmonary exercise testing in recipients of lung and heart-lung transplants demonstrates significant restoration of exercise tolerance to individuals severely disabled by their underlying cardiopulmonary disease. Recipients can perform moderate levels of activity compatible with a normal lifestyle. Considerable exercise limitation, however, remains in most recipients as measured by maximum oxygen uptake and work rate, despite substantial improvement and often normalization in resting cardiopulmonary function. The amount of exercise limitation observed in recipients of single-lung, bilateral-lung, and heart-lung transplants is interestingly similar, and the pattern of limitation is somewhat stereotyped. Ventilatory abnormalities are never limiting. Gas exchange abnormalities are sometimes seen (especially in single-lung transplant recipients) but generally are not limiting. Cardiac dysfunction is sometimes seen (particularly in heart-lung transplant recipients) but also does not appear to be limiting. Peripheral factors limiting exercise (which may include abnormalities in the peripheral circulation and peripheral neuromuscular structure and function) are almost universally seen and are probably the primary determinant of exercise limitation in these patients. At present, the relative contributions of various peripheral factors to exercise limitation are unclear. Further study may help elucidate these issues.