A 5-year-old Japanese boy died because of a ruptured left coronary artery aneurysm (CAA). He was diagnosed as having Kawasaki disease (KD) on the 5th day from onset, with all of the principal signs. On the 7th day of illness, bilateral CAAs were already found via echocardiography, and he was treated with intravenous (IV) gamma globulin and oral ASA. However, the fever persisted and the CAA progressed rapidly. Echocardiography on the 12th illness day showed a giant (18-mm) left anterior descending (LAD) artery aneurysm. Oral propranolol and nifedipine were administered, in conjunction with warfarinaspirin anti-coagulation therapy. On the 13th day of illness, cardiac arrest developed abruptly, and, despite cardiopulmonary resuscitation (CPR), the patient remained unresponsive and died one hour later. The final pathological diagnosis was a ruptured LAD artery aneurysm and cardiac tamponade. Microscopic investigation of the ruptured vascular wall revealed marked neutrophilic infiltration, with fewer macrophages and lymphocytes. CAA ruptures are a very rare, but fatal, complication of KD. Based on a review of previous reports on CAA ruptures, we consider it useful to distinguish aneurysms which rapidly dilate and continue to expand beyond a diameter of 10 mm with ongoing vasculitis (these CAAs can be termed “super-giant”) from the more common giant CAAs limited to a diameter of 8 or 9 mm, because a decision must be made as to whether to start intensive care or to intervene surgically, in order to ensure the survival of patients with such a potentially critical complication. A 5-year-old Japanese boy died because of ruptured left coronary artery aneurysm (CAA). He was diagnosed as having Kawasaki disease (KD) on the 5th day from onset, with all of the principal signs. On the 7th day of illness, bilateral CAAs were already found via echocardiography, and he was treated with intravenous (IV) gamma globulin and oral ASA. However, the fever persisted and the CAA progressed rapidly. Echocardiography on the 12th illness day showed a giant (18-mm) Oral propranolol and nifedipine were administered in conjunction with warfarinaspirin anti-coagulation therapy. On the 13th day of illness, cardiac arrest developed abruptly, and, despite cardiopulmonary resuscitation (CPR), the patient remained unresponsive and died one hour later. The final pathological diagnosis was a ruptured LAD artery aneurysm and cardiac tamponade. Microscopic investigation of the ruptured vascular wall revealed marked neutrophilic infiltration, with fewer macrophages and lymphocytes. CAA ruptures are a very rare, but fatal, complication of KD. Based on a review of previous reports on CAA ruptures, we consider it useful to distinguish aneurysms which rapidly dilate and continue to expand beyond a diameter of 10 mm with ongoing vasculitis (these CAAs can be termed “super-giant”) from the more common giant CAAs limited to a diameter of 8 or 9 mm, because a decision must be made as to whether to start intensive care or to intervene surgically, in order to ensure the survival of patients with such a potentially critical complication.