 This is a prospective, randomized study of patients with single ventricle congenital heart disease presenting for superior cavo-pulmonary anastomosis, or Glenn operation. Such patients in the past have routinely undergone pre-operative invasive cardiac catheterization to assess anatomic and hemodynamic suitability for this procedure. Recent retrospective reviews, including our own J Thorac Cardiovasc Surg 2003: July;1261:272-81 have suggested that such catheterization may not benefit many patients, and that non-invasive assessment may allow adequate evaluation while avoiding many typically minor complications seen with cardiac catheterization in this high risk group of infants. Cardiac MRI is a non-invasive imaging modality that can deliver superior anatomic information such as vascular and intracardiac anatomy as well as functional data ventricular volumes and ejection fraction, and may be a safe alternative to catheterization in such patients. In this study, patients undergo screening echocardiogram prior to enrollment in the study to assess degree of risk for the operation such as pulmonary vein obstruction, a clear risk factor for poor outcome or for demonstrated need for catheterization intervention such as aortic arch obstruction requiring balloon dilation; patients with low risk echocardiograms whose parents grant informed consent for the study are then randomized in a prospective fashion to cardiac MRI or cardiac catheterization. The findings of each study are then reviewed by the subjects cardiologist and cardiac surgeon; patients in whom further pre-operative information may cross over to another study if deemed necessary such as catheterization in patients in whom an unsuspected finding is noted, or cardiac MRI in patients in whom further functional data may be desired. Patients are followed for details of the hospital stay including hospital length of stay, complications, hospital charges for the pre-operative testing, as well as operative and post-operative outcomes newmissed diagnoses, hospital stay, operative and post-operative complications, achievement of a clinical definition of a good outcome from the Glenn operation. 