The majority of cardiac related deaths are due to ischemic heart disease, with the most common clinical scenario being severe coronary artery atherosclerosis resulting in left ventricular myocardial infarction. However, infarction of other cardiac chambers does occur, and often has specific clinical associations. We report a case of a 70-year-old man who suffered from left atrial infarction that resulted in a transmural rupture of his left atrium. The patient had a history of rheumatic heart disease, mitral valve stenosis, and severe atherosclerotic coronary artery disease. Four days before death, he underwent mitral valve replacement and left circumflex coronary artery bypass. Two days later, he developed atrial fibrillation. On the day of death, he had decreased mental status, questionable seizure activity, hematemesis, ventricular tachycardia, and eventually asystole. At autopsy, he had significant hemopericardium with a fibrinous pericarditis and bilateral hemothoraces (total blood volume: 1250 mL). A 0.1 to 0.2 cm left atrial transmural defect was identified. The prosthetic mitral valve was free of vegetations, and completely intact. Similarly, the left circumflex artery bypass graft was completely patent and unremarkable. Severe calcific atherosclerosis was of his native left circumflex and left main coronary arteries. Microscopic examination revealed acute myocardial infarction of the left atrium at the rupture site. The anatomy of atrial circulation as well as the pathology and consequences of atrial infarction are discussed.