A 62 year old man with angina pectoris was admitted for elective coronary arteriography. This was performed by Judkin's technique and revealed hypertensive but good left ventricular function. Selective catheterisation of the left coronary artery was performed with difficulty, eventually using a right coronary Amplatz 2 catheter (Cordis, USA). Its origin was directed posterior, running forwards on the left side of the aortic root, but having an apparently normal course including left circumflex and left anterior descending branches on reaching the AV groove. Computed tomography with intravenous contrast was performed with a GE Hi Speed Advantage (GE Medical Systems, Milwaukee, Wisconsin, USA) helical scanner using 5 mm collimation. This showed that the right coronary artery arose normally from the right coronary sinus (white arrow) and the left coronary artery arose from the right posterior (non-coronary) sinus (black arrow). There was no evidence of any ostial abnormality but the artery arose obliquely and ran between the aortic root and the left atrium to reach the normal anatomical position opposite the left sinus.  Aberrant origin of the coronary arteries in those without congenital heart disease is relatively rare and occurs in fewer than 1% of all adult patients  undergoing  diagnostic  coronary angiography. Only one case of true posterior origin of the left coronary artery has been reported. It may be difficult to show angiographically the exact course of the aberrant vessel. Magnetic resonance imaging and computed tomography may be useful in demonstrating the anatomy as in this case.

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