Background: In general, dominant Left coronary artery (LCA) has Left posterior descending artery (LPDA) and AV nodal branch which gives blood supply to AV node. We evaluated whether the dominant LCA (hypoplastic RCA) is prone to have significant AV block during the acetylcoline (Ach) Provocation test when compared with the non-dominant LCA (hypopastic LCA). Methods: This study consisted of 488 consecutive patients (pts; male 246, 50.4 %; mean age, 55.7 yrs 짹 12.1) without significant coronary artery stenosis. We underwent Ach provocation test by injecting incremental doses of 20, 50 and 100 µg to left coronary artery. Significant coronary artery spasm was defined as focal or diffuse severe transient luminal narrowing (>70%) with/without chest pain or ST-T change of EKG. We evaluated the frequency of AV block during the Ach provocatiojn test and the frequency of significant coronary artery spasm. Result: Baseline characteristics were well balanced between the two groups (Group 1; LCA dominancy, n=140pts and Group II, non LCA dominancy, n=348pts). In pts with LCA dominancy with hypoplastic RCA showed significant higher incidence of AV block during the Ach provocation test but the frequency of the significant coronary artery spasm were not different between the two groups (Table). Conclusion: When we do the Ach provocation test in pts with LCA dominancy, we should be cautious of the development of significant AV block for the patient���s safety and the efficacy of the test.