Aortopulmonary fistulas secondary to thoracic aneurysms are difficult to diagnose and few cases are reported in the literature. We describe the case of a patient first admitted to another hospital with a diagnosis of right upper lobe pneumonia. After the patient had been a week in hospital without improving, a computed tomography scan of the chest revealed a saccular aneurysm of the aortic arch measuring 9 cm at its widest diameter. Hemodynamics and respiratory signs deteriorated a few hours after transfer to our hospital. Physical examination revealed a systolic murmur and poor peripheral perfusion. The patient was intubated and inotropic support was provided. Upon insertion of a Swan-Ganz catheter, given suspicion of septic shock, progressive elevation of mixed venous oxygen saturation was observed. Measurement of venous oxygen saturation was 74% in the right atrium and 93% in the pulmonary artery. Left-right shunt was diagnosed and an emergency aortography revealed the aortopulmonary fistula. Emergency surgery was performed with the patient in deep hypothermia and cardiocirculatory arrest. There were no adverse events, and postoperative recovery and clinical course were good.