The authors report the case of a very rare coronary malformation: atresia of the left main coronary artery. This anomaly was discovered in a marathon runner who presented effort angina with a positive exercise stress test. At coronary angiography, it was not possible to catheterise the left coronary artery and only a minuscle dimple could be visualised. Selective right coronary catheterisation showed a very large right coronary artery with retrograde injection of the whole of the left coronary network, as far as the left main stem. The authors did not suggest revascularisation because of the good functional tolerance of this malformation.