Recent data suggest potential benefits of cardiac resynchronization therapy in the management of right ventricular (RV) dysfunction in congenital heart disease. The aim of this study was to determine the nature, prevalence, and functional implications of mechanical RV dyssynchrony in patients after Senning or Mustard procedures for transposition of the great arteries. Twenty-eight patients (mean age 21.1 ± 3.5 years) at 19.9 ± 3.2 years after atrial switch operations and 29 healthy controls were studied. The times from the onset of QRS to peak systolic strain (Tε) at the base of and the mid RV free wall, the ventricular septum (VS), and the left ventricular (LV) free wall were determined using tissue Doppler echocardiography. Intraventricular mechanical delay was defined as ΔTε_RV-VS and interventricular mechanical delay as ΔTε_RV-LV. In patients, the magnitude of RV intra- and interventricular mechanical delay was correlated with cardiac magnetic resonance–derived RV volumes and ejection fractions (n = 26) and treadmill exercise testing parameters (n = 20). Compared with controls, patients had significantly longer ΔTε_RV-VS (48.1 ± 50.9 vs 17.0 ± 16.1 ms, p <0.001) and ΔTε_RV-LV (63.1 ± 49.5 vs 19.0 ± 12.9, p <0.001). Nine patients (32%) exhibited RV dyssynchrony (ΔTε_RV-VS >49 ms, control mean ± 2SD), and 16 patients (57%) showed interventricular dyssynchrony (ΔTε_RV-LV >45ms). In patients, RV intra- and interventricular mechanical delay was correlated negatively with the RV ejection fraction (both r = −0.42, p = 0.03) and percentage predicted maximum oxygen consumption (r = −0.50, p = 0.03, and r = −0.52, p = 0.02, respectively) and positively with minute ventilation/carbon dioxide production slope (r = 0.49, p = 0.03, and r = 0.56, p = 0.01, respectively). In conclusion, RV dyssynchrony is common in young adults after atrial switch operations and is associated with RV systolic dysfunction and impaired exercise performance.