A 58-year-old Japanese woman was admitted to our hospital because of chest pain. A continuous murmur was detected at the left parasternal area. Electrocardiogram showed ST elevation in leads V2, V3 and V4. Chest computed tomography and echocardiography demonstrated pericardial effusion and a large mass which was adjacent to the pulmonary artery. An abnormal blood flow was detected in the mass by Doppler echocardiography. Coronary angiography confirmed that the mass was a giant aneurysm of coronary arteriovenous fistula arising from both the left and right coronary arteries. This patient had no symptoms until rupture of the fistula. Rupture of a coronary arteriovenous fistula is very rare but can be a cause of chest pain and pericardial effusion.