Congenital coronary arteriovenous fistula is an unusual, but not rare, coronary anomaly. Management of asymptomatic fistulas is controversial because of great variability in natural history. We describe an 82-year-old female patient with spontaneous rupture of a previously undetected left main coronary artery-to-pulmonary artery coronary arteriovenous fistula, with resulting hemopericardium and cardiac tamponade. Emergent surgical exploration and repair provided successful treatment.