Background: In transposition of the great arteries (TGA), the right ventricle (RV) is subaortic and abnormal aortic structure or function could adversely affect the capacity of the RV to supply the systemic circulation. Our aim was to assess aortic dimensions and distensibility and RV function in patients with palliated TGA using cardiovascular magnetic resonance imaging (CMR). Methods: We studied 29 patients (22 males; age 29±4 years) with simple TGA, who underwent an atrial switch procedure, and 29 age and sex matched controls. All subjects had cine and phase contrast CMR to evaluate aortic function and global RV function. Results: TGA patients had significant dilatation of the aortic annulus (21.0±3.6 mm vs. 17.6±4.1 mm, p=0.002) and the sinus of Valsalva (30.0±4 mm vs. 26.8±4.2 mm, p=0.005), compared to controls. These findings were associated with reduced distensibility of the ascending aorta in patients with TGA (3.5±1.6 vs. 5.3±2.4 mmHg(-1) x 10(-3), p=0.0009). We could not show a significant correlation between aortic stiffness indices and RV size, function, mass or presence of fibrosis. Conclusion: The aortic root dilates and the ascending aorta stiffens in TGA, during young adult life. Although these proximal aortic changes did not show adverse effects on the RV in our young TGA sample, they might have important long-term physiopathological consequences in these patients.