Thermal coronary angiography is a noninvasive but not yet quantitative method to intraoperatively assess graft patency in CABG surgery. Aim of this study was to quantify graft flow by measuring perfusion-induced myocardial temperature changes over time. Saphenous vein grafts to the left anterior descending artery were perfused at flow rates of 16-105 ml/min with warm saline. A thermal scanner with a 256 x 256 focal-plane array detector providing a spatial resolution of 1.2 mrad was used. The resulting temperature curves were averaged and a non-linear fit procedure was performed to calculate the time constant (tau) at each flow rate. An increase of myocardial temperature along the LAD with different flow rates could be demonstrated. There was an excellent correlation between the calculated time constant and actual flow (r = 0.96, p < 0.0002). By determining the time constant for different flow rates an estimate of actual graft flow is possible using thermal coronary angiography. Clinical studies have to show if the time constant can be used as a predictor of graft flow in patients.