Twenty-two late survivors of Mustard repair of transposition of the great arteries underwent treadmill exercise testing to assess exercise endurance, and radionuclide cineangiography to measure rest and exercise right ventricular (RV) and left ventricular (LV) ejection fraction (EF). Mean age at Mustard repair was 1.8 +/- 2.4 years and at initial exercise study 14.1 +/- 4.5 years. All patients were asymptomatic at study entry. Treadmill endurance time (9 to 12 minutes) was within the normal range for age in 18 of 20 patients. LVEF and RVEF at rest were within normal limits (55 +/- 8% and 52 +/- 9%, respectively). All LVEFs and RVEFs during exercise were also within the normal range (64 +/- 8% and 57 +/- 9%, respectively). However, the magnitude of increase in EF (rest to exercise) was subnormal for the right ventricle in 7 patients and for the left ventricle in 2 patients. Exercise RVEF was higher in patients with simple transposition who underwent repair at < 1 year of age than in those who underwent operation at age > 1 year (62 +/- 10% vs 52 +/- 7%). Serial study in 6 patients revealed no change in average RVEF or LVEF after an average interval of 4.4 years. One patient with complex transposition subsequently developed refractory congestive heart failure requiring cardiac transplantation. It is concluded that exercise endurance and LVEF and RVEF at rest and exercise are generally well preserved up to 19 years after Mustard repair, particularly in cases of simple transposition of the great arteries repaired before 1 year of age.