Background: Anomalous aortic origin of a coronary artery (AAOCA) with an interarterial course is associated with sudden cardiac death in children. Objectives: Using cardiac MRI with adenosine, we evaluated coronary ostial stenosis, proximal coronary size, and left ventricular (LV) function in children with AAOCA. Methods: We prospectively enrolled children 5-18 years old with interarterial AAOCA. MRIs were reviewed for coronary artery origin, proximal course, dimensions, and cardiac function. Surgery consisted of the modified unroofing procedure. We used descriptive statistics and paired t-tests to evaluate for statistical significance. Results: Between 2/2009 and 5/2014, 24 subjects with AAOCA underwent 29 MRIs. The majority were male (N=19, 79%) with anomalous right coronary artery (AAORCA, N=20, 83%). Mean age was 12.8 years at time of initial MRI. MRI was performed an average of 7 months post-operatively in 8 subjects who underwent surgery. In all, the proximal anomalous coronary arose tangential to the aorta with an elliptical, slit-like ostium. The anomalous coronary measured smaller proximally (0.20 mm) compared to distally (0.31 mm, P=< 0.0001), and after surgical repair, the post-operative origin was significantly larger (0.36 vs. 0.21 mm, P=0.02). Other abnormalities at initial MRI included fixed inferior wall (N=1) and reversible subendocardial septal/inferior wall (N=1) perfusion defects. Post-operatively, the neo-ostium was round in 6 (see Figure), but in 2, the orifice remained elliptical. One patient had a new small mid-myocardial scar and one had dyskinetic septal wall motion. LV function was normal both before and after surgery (mean ejection fraction =68.1% vs. 67.5%, P=0.85). Conclusions: Cardiac MRI with adenosine is an important tool for the evaluation of anomalous anatomy, myocardial function, and ischemia/injury and should be considered for the initial and, when applicable, post-operative assessment of children with AAOCA.