Coronary artery fistulae are anomalies characterized by one or more abnormal communications between the coronary arteries and another part of the circulatory system. Haemodynamically they function as arteriovenous shunts, causing blood to bypass the capillary bed. Most of them are not, however, genuine arteriovenous communications in the strict sense of the term, since they empty into a heart chamber or into the pulmonary artery.The anomaly is rare. So far only about 100 cases have been described. The aetiology, physiology, and anatomical variations are reviewed in recent publications by Gasul, Arcilla, Fell, Lynfield, Bicoff, and Luan (1960) and Upshaw (1962). The condition has lately become of increasing interest since it can be diagnosed by cardiac catheterization and coronary angiography, and most cases are amenable to surgical treatment at an acceptable risk.This paper reports three cases with some remarks on the physiology of the condition and its treatment. CASE REPORTS CASE 1 M. P., a 44-year-old woman, was admitted in 1957. From the age of 20 she had been troubled by palpitations and dyspnoea on exercise. For the last six months she had had increasing dyspnoea and retrosternal pain. The heart action was regular but there were some extrasystoles. Blood pressure was 140/90 mm. Hg. A high-frequency, continuous murmur was heard over the heart, maximal at the left lower sternal border. The E.C.G. was normal and radiographic examination revealed a relative heart volume of 390 ml./m.2 and some enlargement of the right atrium and ventricle. Right heart catheterization showed normal pressures in the right heart and pulmonary artery. A left-to-right shunt to the pulmonary artery was demonstrated and calculated to 2-9 I./min., or 38-6% of the total pulmonary flow of 7.5 1./min. At thoracotomy a plexus of vessels was found to connect the right coronary artery and the pulmonary artery (Fig. 1). A thrill was felt in the area. The plexus was temporarily clamped without a change in the heart rhythm or the E.C.G. The fistulous plexus was closed with multiple ligatures both at its coronary origin and close to the pulmonary artery. The thrill disappeared but the heart rhythm and E.C.G. remained unchanged. The post-operative course was uneventful. On examination three months later the patient had no dyspnoea, palpitations, or precordial pain. The E.C.G. was normal. A faint systolic as well as a faint diastolic murmur (but no continuous murmur) were heard over the heart. From 1959 on she again developed dyspnoea and anginal pain on exertion, but the symptoms were moderate and progressed very little during the following years. On admission in 1965 the heart action was regular. Blood pressure was 145/95 mm. Hg. A high-frequency, continuous murmur of moderate intensity was again heard at the left sternal border. An apical systolic murmur was also audible. The E.C.G. showed a digitalis effect but was otherwise normal. On radiological examination the heart contours were normal and the relative heart volume was calculated to 450...