Blood flow in 226 aortocoronary vein bypass grafts in 125 patients was measured with an electromagnetic flow meter. Blood flow occurred primarily in diastole. Mean flow for each of the three main coronary arteries was 63 cc/min with a wide range for individual coronary arteries. The higher ranges of flow could be obtained if (a) bidirectional flow could be established with an end-to-side vein-coronary anastomosis in the presence of a proximal obstruction, and (b) the myocardium was normal. With optimal conditions, the flow in vein grafts will increase with time.