As an increasing number of patients with congenital heart disease reach adulthood, more information is needed regarding outcomes. The first signs of impaired heart function may appear during exercise testing. The aim of the present study was to establish mean values for maximal oxygen uptake in adults with various congenital heart diseases. Patients from 6 major diagnostic groups were identified, including patients with atrial septal defect (ASD, n = 93), transposition of the great arteries corrected with the Mustard procedure (n = 84), congenitally corrected transposition of the great arteries (CCTGA, n = 41), Tetralogy of Fallot (n = 168), Ebstein's anomaly (n = 37), and Modified Fontan procedure (n = 52). Diminished maximal oxygen uptake was found in all diagnostic groups across age compared with healthy subjects. A significant decrease in maximal oxygen uptake with aging was found in those with ASD (p <0.0001), CCTGA (p = 0.01), and Tetralogy of Fallot (p <0.0001). There was no significant decline, however, in Ebstein's anomaly (p = 0.270), Fontan procedure (p = 0.182), and in the Mustard patients (p = 0.188). All patients achieved significantly lower heart rates than predicted (mean for all groups, p <0.0001). Forced vital capacity values (3.51 L, mean SD +/- 1.02) were lower than predicted values (4.10 L, mean SD +/- 0.90, p <0.0001) for all patients groups except those with ASD. Mean values, however, were within the accepted 20% range of variance. This study showed diminished aerobic capacity in all diagnostic groups when compared with a healthy population. The maximal oxygen uptake values across age groups can be used as reference values in patients with similar diagnoses and as the basis for further research.