Coronary flow reserve provides a gold standard assessment of the epicardial and microvascular coronary circulation. However, measurement of coronary flow reserve is limited by the invasiveness or complexity of the methods hitherto available. We investigated whether transthoracic echocardiography could be used to assess coronary flow reserve. We imaged distal left anterior descending coronary artery diameter and flow in 14 healthy volunteers, both at rest and during intravenous infusion of adenosine (140 microg/kg per minute). Volunteers were men, with an average (+/-SD) age of 28.4 +/- 6.3 years. Complete data were acquired in 11 cases. Average distal left anterior descending coronary artery diameter was 0.213 +/- 0.03 cm. Velocity time integral rose from 8.6 +/- 2.1 cm to 27.7 +/- 5.6 cm with adenosine infusion. Heart rate rose from 64.7 +/- 9. 8 to 75.3 +/- 11.7 bpm. The Doppler angle of incidence to flow was 42.4 +/- 8.7 degrees. Resting distal left anterior descending coronary artery flow was therefore calculated as 13.4 +/- 3.2 mL/min and hyperemic flow as 51.2 +/- 16.2 mL/min, yielding a coronary flow reserve of 3.81 +/- 0.6. We conclude that coronary flow reserve can be assessed in a selected population with the use of transthoracic echocardiography and an intravenous infusion of adenosine. The simplicity of this noninvasive technique suggests that it could become a useful tool for measurement of coronary flow reserve if imaging success rates can be optimized.