There has been no in vivo validation of the use of transthoracic echocardiography to measure distal left anterior descending coronary artery (LAD) diameter. We therefore undertook transthoracic echocardiography on 65 male patients immediately before cardiac catheterization to compare echocardiographic and angiographic findings. The distal LAD was successfully imaged in 41 (63%) patients; 29 of these had an angiographically normal distal LAD as assessed by an independent cardiologist and formed the study group. Transthoracic echocardiographic and quantitative coronary angiographic measurements of distal LAD diameter were made. Echocardiographic measurements ranged from 0.14 to 0.28 cm (mean 0.20 cm). Angiographic results ranged from 0.12 to 0.28 cm (mean 0.195 cm). Correlation between techniques was good (r=.925). The maximum discrepancy between transthoracic echocardiography and quantitative coronary angiography was 0.03 cm. Limits of agreement were +0.032 to -0.024 cm. We conclude that transthoracic echocardiography is a valid technique for measurement of distal LAD diameter.