Background As coronary flow velocity (CFV) is inversely related to the luminal size that exists for the myocardial bed, the elevated arterial tone can be assessed as the higher flow velocity in the epicardial artery. We examined the usefulness of transthoracic Doppler echocardiography (TTDE) for the assessment of coronary arterial tone. Methods and Results A total of 32 patients underwent TTDE and angiography. The luminal diameter (LD) in the left anterior descending artery (LAD) was measured by using quantitative coronary angiography before and after nitroglycerin (NTG) administration. The ratio of post NTG LD to the control (LD_NTG/Pre) was assessed as a standard parameter of coronary arterial tone. We also measured CFV and CFV reserve (CFVR) at the LAD by TTDE. We evaluated the change of CFV (CFV_NTG/Pre) and CFVR (CFVR_NTG/Pre) following NTG administration. The LD increased from 1.98±0.46 to 2.51±0.34 mm (p<0.001), while the CFV decreased from 23.9±10.0 to 16.3±5.6 cm/s (p<0.03), and the CFVR increased from 2.39±0.65 to 3.56±1.12 (p<0.001). There were significant correlations between CFV_NTG/Pre and LD_NTG/Pre (p<0.0001, R² =0.532), and between the CFVR_NTG/Pre and LD_NTG/Pre (p<0.0001, R² =0.715). Conclusion TTDE can assess the coronary arterial tone by measuring the responses of CFV and CFVR to NTG administration. (Circ J 2006; 70: 459 - 462)