Background: Coronary endothelial dysfunction (CED) and subsequent clinically significant coronary artery spasm (CAS) at the myocardial bridge (MB) segment has known to be associated with adverse cardiac events. We evaluated whether there are differences in clinical and angiographic characteristics between MB patients (pts) with CAS and without CAS as assessed by acetylcholine (Ach) provocation test. Methods: A total 341consecutive pts (194 men; mean age 56.08짹12.57 years) with MB underwent Ach provocation test by incremental doses of 20, 50, 100 ug into the left coronary artery. The Ach provocation test results and its associated parameters were compared between the MB pts with positive provocation group (CAS+, n=263) and negative provocation group (CAS-, n=88). Results The baseline clinical characteristics were similar between the two groups except more elderly pts (52.97 짹12.42 vs. 57.12 짹 12.04, p=0.006) in the CED group. MB with CAS group showed higher incidence of ischemic chest pain, ST-T change, baseline spasm, more severe milking effect in MB and AV block during the Ach test compared with those of MB with CAS (-) group. Multivariate logistic analysis showed that age, typical chest pain, (+) response to lower (A1) Ach dose, higher baseline spasm, higher angiographic severity of MB and more AV block during provocation test were significantly associated with significant CAS in MB pts (Table). Conclusion: In our study, MB pts with CAS was associated with older age, more chest pain, provocation positive at lower Ach dose, baseline spasm, severe MB and presence of AV block as compared with those of MB pts without CAS.