A coronary artery fistula is an abnormal communication between a coronary artery and a cardiac chamber, great vessel, or other vascular structure. It is an infrequent but potentially important abnormality that can affect any age group. Most are congenital in origin, although other etiologies, in particular trauma, have been identified. Many are small and found incidentally during coronary arteriography, while others are identified as the cause of a continuous murmur, myocardial ischemia, congestive heart failure, or, rarely, bacterial endocarditis. The diagnosis should be considered in any patient presenting with a continuous murmur or in the setting of congestive heart failure, myocardial ischemia, or bacterial endocarditis without an obvious etiology. The pathophysiologic mechanisms resulting in symptoms include cardiac volume overload due to the shunting of blood and reduction of the myocardial blood supply due to "coronary steal." The diagnosis of coronary artery fistula may be suggested by the finding of a continuous murmur in a precordial location, which is atypical for patent ductus arteriosus. Two-dimensional echocardiography may demonstrate dilated coronary arteries, and pulse-wave and color-flow Doppler examinations can display turbulent flow in the suspected fistula and its receiving chamber or vessel. Angiography is generally needed to confirm the diagnosis and elucidate anatomic detail. The natural history of coronary artery fistula is variable, with long periods of stability in some patients and gradual progression of symptoms in others. Small fistulas detected incidentally at the time of angiography do not require immediate surgical correction, but careful follow-up is indicated because the potential for enlargement with physiologically important shunting exists and cannot readily be predicted. Spontaneous closure is uncommon. Surgical repair of the fistula is recommended for symptomatic patients and for some without symptoms because a quantitatively small shunt does not predict freedom from future symptoms or complications. Those selected for medical management must be followed closely.