This paper describes an 8.5-year-old male castrated Greyhound with a sudden onset of a grade IV/VI continuous murmur. Five days before presentation to Cornell University’s small animal emergency service, the patient was noted to become acutely lethargic, began intermittently coughing, and was inappetent. Two days later the patient was taken to the referring veterinarian and a heart murmur was observed. This murmur was not ausculted at an appointment two weeks prior. Thoracic radiographs suggested left-sided heart failure and enalapril and furosemide were started. The patient was then referred to Cornell after stabilization. A consult with the Cardiology service showed continuous flow from the left aortic sinus into the pulmonary artery on echocardiogram. A diagnosis of an aorticopulmonary fistula was made due to a presumptive aortic aneurysm of the left aortic sinus that ruptured into the main pulmonary trunk. He was treated with a continuous rate infusion of furosemide and discharged to the care of his owners on the following day.