An 11-year-old boy developed an acute aortic pseudoaneurysm during balloon dilation of unoperated coarctation of the aorta. The pseudoaneurysm occurred despite the fact that the balloon catheter size did not exceed the diameter of the aorta at the diaphragm. Elective delayed surgical repair was successfully performed after 3 weeks of antihypertensive treatment when serial magnetic resonance imaging had demonstrated a reduction in the local tissue swelling.