This study sought to determine if areas of late gadolinium enhancement (LGE) would be present in adults with systemic right ventricles and if LGE would be associated with markers of adverse outcomes. Using gadolinium-enhanced magnetic resonance and cardiopulmonary exercise testing, 34 adults with systemic right ventricles (23 patients with atrial repair for transposition of the great arteries and 11 with congenitally corrected transposition) were studied at a mean age of 25 years. LGE was present in 14 patients (41%). The presence of LGE was associated with older age (p = 0.037), a lower right ventricular (RV) ejection fraction (34% vs 45%, p = 0.006), higher RV wall stress (p = 0.0001), reduced peak oxygen uptake (47% vs 56%, p = 0.001), and a history of arrhythmia (p = 0.005). The RV ejection fraction was correlated with RV wall stress (r = -0.81, p <0.0001) and peak oxygen uptake (r = 0.74, p <0.0001). Twelve patients experienced worsening of their clinical conditions. This was associated with decreases in biventricular function and increases in the prevalence and number of LGE areas. In conclusion, patients with systemic right ventricles have areas of abnormal myocardium, presumably due to fibrosis, that can be seen by contrast-enhanced magnetic resonance imaging. The presence of abnormal myocardial regions is associated with RV dysfunction, poor exercise tolerance, arrhythmia, and progressive clinical deterioration.