The exercise capacity of children after arterial switch for transposition of the great arteries (TGA) is known to be at the lower limit of normal. We aimed to ascertain whether this results from compromised hemodynamics or deconditioning. A total of 17 children with TGA (12 male and 5 female children; age 12.1 ± 2.0 years) treated with the arterial switch operation were compared with 20 age-matched controls (13 male and 7 female children; age 12.8 ± 2.4 years) regarding their peak exercise capacity, peak workload, and peak heart rate, as assessed by cycle ergometry. The children's physical activity level was monitored for a 7-day period using a pedometer and diary, and a questionnaire was used to assess physical activity participation and overprotection. The results demonstrated that TGA children showed a significantly reduced peak exercise capacity (47.4 ± 6.4 vs 41.1 ± 6.6 ml/kg/min; p <0.05), maximal workload (3.7 ± 0.5 vs 3.1 ± 0.6 W/kg; p <0.01), and maximal heart rate (189 ± 9 vs 180 ± 14 beats/min; p <0.05) compared to the controls. No significant differences were found in the physical activity pattern or overprotection. In conclusion, given the comparable physical activity level, but reduced exercise capacity in the TGA children, these children most likely fall short in their exercise performance because of restrictive hemodynamics rather than deconditioning from reduced daily life activity.