Background: Coronary artery ectasia (CAE) has been known to be associated with coronary morbidity but consensus on its incidence and clinical significances especially relationship with coronary artery spasm is not clearly defined. We investigated the incidence of CAE and its relationship with endothelial dysfunction. Methods: The study consisted of 696 patients (pts, Male 382, Age 59.2짹8.1 years) who underwent diagnostic coronary angiography. Among them, a total 400 pts underwent acetylcholine (Ach) provocation test by injecting incremental doses of 20, 50 and 100 關g to left coronary artery. CAE was defined an abnormal dilatation of coronary artery wall greater than 1.5 times of normal coronary artery. Results: The incidence CAE was found in 2.0% (14/696) in study population. Among the 14 CAE pts, 9 pts (Male 7, Age 56짹11.2 years) received Ach provocation test due to typical history of variant angina. Typical severe spasm was induced in 77.7% of the pts (7/9) with either lower dose of Ach (50 關g, 57.1%, 4/7) or higher dose of Ach (100 關g, 42.9%, 3/7). Only one pt had diffuse severe spasm throughout the whole coronary artery including ectatic segment (1/7, 14.3%) and most of severe spasms were observed mainly at mid or mid to distal left anterior descending artery (LAD, 6/7, 85.7%). The severe spasms were frequently found between the borders of ectatic portion and normal portions. Fixed coronary lesions were observed in 28.6% (2/7) and typical myocardial bridging in mid LAD was observed in 28.6% (2/7), too. Typical chest pain was reproduced in 57.1 % (4/7) without obvious ST-T change. Conclusion: The incidence of CAE in a routine clinical practice is 2%. Greater than 77% of pts had Ach induced endothelial dysfunction and significant proportion of CAE pts might be associated with coronary vasospasm and related coronary morbidity. This CAE associated endothelial dysfunction was frequently observed between the borders of ectatic and normal coronary artery segment.