Background: Previous reports have suggested a link between myocardial bridge (MB) and coronary spasm. Endothelial dysfunction and/or dysfunction of smooth muscle cells are considered to be involved in coronary spasm at MB site. We evaluated the incidence of significant endothelial dysfunction in patients (pts) with MB and clinical characteristics according to angiographic severity of MB. Methods: A total 141 pts (82 men, Age; 54.4짹11.3 years) who underwent diagnostic coronary angiography with typical angiographic ���milking effects��� were selected for incremental acetylcholine (Ach) provocation test (A1 to A3, 20, 50 and 100關g). Qualitative coronary angiography (QCA) analysis was done for assessing MB severity. Results: Ninety eight percent of the pts had MB at left anterior descending artery (LAD). Angiographic and clinical characteristics during Ach provocation test according to MB severity are shown on the table. Overall, 74.5% (105/141) of MB pts had (+) Ach provocation test. The proportion of (+) provocation, proportion of pt���s response to lower dose of Ach and ST-T change during provocation test was increased according to MB severity. Spasm patterns such as focal or diffuse and pt���s subjective symptoms were not associated with MB severity. Conclusion: Greater than 70% of pts in MB had significant endothelial dysfunction by Ach provocation test and this could be responsible for the pt���s coronary morbidty related to coronary spasm. The more severe MB, the more pts had (+) provocation results, (+) response to lower Ach dose and ST-T change in EKG during the provocation test.