Invasive clinical assessment of endothelial-dependent coronary microvascular function currently relies on intracoronary Doppler flow measurements and quantitative coronary angiography (QCA) to derive changes in coronary blood flow in response to endothelial agonists [1]. However, this method can be technically challenging with poor reproducibility. We hypothesised that changes in coronary flow derived by a thermodilution method, using the pressure wire (which can function as an intracoronary dual pressure-temperature sensor), could be used to reliably assess coronary microvascular endothelial function. The transit time (Tmn) of a bolus of room temperature saline using the latter technique is known to be inversely proportional to coronary flow [2]. Twenty patients undergoing PCI to a single vessel were recruited and an adjacent coronary vessel free of significant disease was studied. We compared the percentage change in absolute coronary flow from baseline using Doppler/QCA with the percentage reduction in Tmn using thermodilution in response to a 2 min intracoronary infusion of substance P (20pmol/min). Thermodilution and Doppler derived values were compared using linear regression analysis. There was a very close correlation (R²=0.75, p=0.0001) between percentage change in absolute blood flow in response to substance P (as measured by a Doppler flow wire) and reduction in Tmn (thermodilution) as measured with the pressure wire. This simple thermodilution technique can be used to reliably assess endothelium-dependent changes in coronary blood flow and hence microvascular endothelial function. The results correlate well with the current gold standard using the Doppler flow wire.