Patients recovering after heart valve surgery are not in an optimum state of 'cardiorespiratory fitness'. To examine this proposition a controlled trial of physical training was undertaken in patients recovering from the replacement of a single heart valve. Patients were allocated to a test or control group two weeks after operation. Each patient performed a submaximal exercise test at entry, and twelve and twenty-four weeks after this test. The Canadian Air Force exercise programme was undertaken by the test group, while the control group continued normal activities for the twenty-four weeks between the first and last exercise test. A regression line of submaximal heart rate on oxygen consumption was calculated from the data of each exercise test in each patient. Alterations in this line were used as an 'index' of changes in 'cardiorespiratory fitness'. The individual results showed a consistent improvement in 'cardiorespiratory fitness' over the first 12 weeks in both groups. Only patients in the test group continued to improve between 12 and 24 weeks. Thus the exercise programme modified the recovery of 'cardiorespiratory fitness' after operation. A physical fitness rehabilitation programme may help these patients gain maximum benefit from correctiv surgery.