Greater numbers of children with congenital heart disease are surviving to adulthood. The non-invasive assessment and surveillance of these patients, still based primarily on transthoracic echocardiography, has been significantly enhanced by the advent, and more widespread use of, cardiac magnetic resonance imaging. We report on the influence of cardiac magnetic resonance imaging in the initial evaluation of, and response to treatment in, a patient who had developed an obstruction within her Fontan circuit.