A case is described in which a giant aneurysm developed following successful PDA coil occlusion in an infant with Marfan syndrome. This rare and severe complication brings into question the need for careful evaluation of these children before and after transcatheter occlusion. Cathet. Cardiovasc. Intervent. 50:186–189, 2000. © 2000 Wiley‐Liss, Inc.