Background: Coronary endothelial dysfunction (CED) and subsequent clinical spasm at the myocardial bridge (MB) segment has been proposed as a risk factor for adverse cardiac events. There is limited data regarding clinical and angiographic characteristics according to CED severity in patients (pts) with MB. Methods: A total 247 consecutive pts (141 men; mean age 57.01짹11.87 years) with MB underwent acetylcholine (Ach) provocation test by incremental doses of 20, 50, 100 ug into the left coronary artery were enrolled from March 2004 to April 2009. Severe CED was defined as focal or diffuse severe transient luminal narrowing (>70%) with/without chest pain or ST-T change on ECG. The Ach provocation test results and its associated parameters were compared between the mild (<50%)/moderate (50-70%) CED group (n=121) and severe CED group (n=126). Results Baseline characteristics were similar between the two groups. The incidence of ischemic chest pain and ST-T change were not different between the two groups. The mild/moderate CED pts tended to need higher Ach dose (A3) to get final results. Pts with severe CED showed more severe angiographic milking effect (severe narrowing in MB segment), higher incidence of baseline spasm, and more severe angiographic narrowing to Ach infusion. However, the incidence of diffuse and multivessel CED was not different between the two groups (Table). Multivariate logistic analysis showed that presence of higher MB % , mean % narrowing and baseline spasm was still significantly associated with severe CED. Conclusion: In our study, greater angiographic narrowing percentage of MB segment and presence of baseline CED before Ach provocation test were associated with severe CED in pts with MB.