Background and Purpose: In acute coronary syndrome (ACS), the most potential cause is thrombotic occlusion arising from ruptured vulnerable plaque based on coronary atherosclerosis, whereas coronary artery spasm (CAS) is another well-defined mechanism.We investigated clinical features of CAS in ACS in the real world.Methods: We retrospectively extracted consecutive suspected ACS patients in March 2006-July 2010 (n=720), based on symptoms and ischemic ECG changes and/or cardiac biomarkers.Among patients undergoing selective coronary angiography within two weeks after admission (n=655), patients with past CABG (n=20) were excluded, and the following two subgroups were finally selected; Organic (O): culprit lesion*3f75% organic stenosis with/without thrombotic occlusion, Spastic (S): either 1) organic stenosis<75% with positive acetylcholine test, or 2) organic stenosis<75% without acetylcholine test, but with transient ischemic ECG changes relieved by sublingual nitroglycerin.We compared clinical characteristics between the two groups.Results: The prevalence and in-hospital mortality of S were 59/635 (9.3%) and 0/59 (0%), and those of O were 516/635 (81.3%) and 17/516 (3.3%), respectively.In S, multivessel spasm was provoked in 31/42 patients (73.8%), myocardial infarction occurred in 11/59 patients (18.6%), and 2/59 patients (3.4%) experienced aborted sudden cardiac death.S was younger than O without significant sex distribution.The prevalence of hypertension, hypercholesterolemia, and diabetes mellitus were lower in S than in O, but that of smoking was not significantly different between the two groups (Table).Conclusions: CAS is one of the potential causes of ACS.Initial differential diagnosis is important in the treatment strategy of ACS.Smoking is a relatively potent risk factor in ACS caused by CAS than by significant organic stenosis.