Background: Coronary artery spasm due to significant endothelial dysfunction can be an important cause of myocardial ischemia. Intravascular ultrasound (IVUS) is a useful device to evaluate the coronary vessel wall and associated plaque characteristics and distribution but the IVUS characteristics in patients (pts) with acetylcholine (Ach)-induced significant endothelial dysfunction are not well known. Methods: A total of 110 pts presented with suspected variant angina underwent coronary angiography, Ach provocation test (20, 50 and 100 µg to left coronary artery) and subsequent IVUS examination. Significant coronary artery endothelial dysfunction was defined as focal or diffuse severe transient luminal narrowing (>70%). IVUS characteristics of Provocation (+) and (-) pts were compared. Results: Of the 110 pts enrolled in the study cohort, more than half of the pts showed significant endothelial dysfunction (55.5%, 61/110). Baseline clinical characteristics were similar between the two groups except the higher incidence of hypertension in provocation (+) group (66.7% vs 42.9%, P=0.010). The majority of plaques in spasm location were eccentric and of low echogenicity. There were no significant differences in plaque size, morphology and distribution between two groups (table). Conclusion: In our study, most of the plaques in coronary artery spasm segment were soft and eccentric. Contrast to previous studies, it seemed that plaque size and morphology had no adverse impact on the incidence of significant coronary endothelial dysfunction induced by Ach.