Immunoglobulin G4 (IgG4)-related disease is rare. It is characterized by marked elevation in serum IgG4 concentration and infiltration of IgG4-positive plasma cells into a variety of tissues,1 particularly the adventitia surrounding great arteries. Here, we report the case of a patient who suffered an acute myocardial infarction and eventually died of a ruptured thoracic aorta as a result of IgG4-related disease. An 84-year-old man was admitted to our hospital complaining of new-onset substernal compressing pain. The diagnosis of acute posterolateral myocardial infarction was made by means of ECG, ultrasound imaging, and the presence of elevated serum creatine phosphokinase and troponin-I levels. Subsequently, urgent coronary angiography showed a huge coronary aneurysm (21 mm at its maximal diameter) accompanied by thrombus in the left circumflex artery (Figure 1 and online-only Data Supplement Movie I). Insignificant stenosis could be seen proximally in several branching arteries, and the right and left anterior descending arteries appeared patent (Figure 2A and online-only Data Supplement Movie II). Thrombus was successfully aspirated from the left circumflex artery, and a stent was subsequently implanted. Figure 1. Left circumflex coronary artery (LCX) images obtained by invasive coronary angiography (right anterior oblique caudal view). A huge coronary aneurysm (21 mm at its maximal diameter) is evident (dotted arrows), with thrombus at the distal end (arrow). See online-only Data Supplement Movie I. Figure 2. Right coronary artery (RCA) images obtained by invasive coronary angiography, coronary computed tomography angiography (CCTA), and intravascular ultrasound (IVUS) imaging. A , RCA image (left anterior oblique view). There was no significant stenosis. See online-only Data Supplement Movie II. B , Curved multiplanar reconstruction image …