Background Aspirin is a non-selective COX inhibitor and high dose aspirin can aggravates coronary artery spasm (CAS). However whether low dose aspirin can similarly impact on coronary spasm is largely unknown. We evaluated the impact of low dose asprin on acetylcholine (Ach) induced coronary artery spasm (CAS) and 12 month clinical outcome. Methods A total 1286 consecutive patients without significant coronary artery disease who had taken low dose aspirin (less than 100mg) and underwent the Ach provocation test by injecting incremental doses of 20, 50, 100 ug into between November 2004 and March 2009.The Ach provocation test results and 12 clinical outcomes were compared between the Aspirin group (n=287) and non-aspirin group (n=999). Results The baseline clinical characteristics showed that the classic risk factors including diabetes mellitus (20.6% vs 7.3%, p<0.01), hypertension (59.3% vs 35.9%, p<0.01), hyperlipidemia (21.6% vs 14.9%, p=0.01) and smoking (34.8% vs 28.5%, p=0.04) were more common in the aspirin groups. After Ach injection, the rate of positive provocation test result, chest pain, diffuse and severe CAS pattern were more common in the Aspirin group (Table). Multivariate logistic analysis showed that aspirin itself was an independent risk factor for Ach induced CAS (Adjusted odd ratio; 1.28, 95% confidence interval; 1.01-1.62, p=0.037). 12 month clinical outcomes including death, myocardial infarction (MI) were similiar between two groups. However, repeat spasm test due to recurrent chest pain was more common in aspirin group (2.8% vs 0.2%, p<0.01). Conclusion In our study, low dose aspirin was an independent risk factor of Ach induced significant CAS and associated with diffuse spasm and repeat spasm test due to recurrent chest pain. However,12 month clinical outcome including death and MI was not associated with aspirin. Special caution should be regarding even low dose aspirin prescription in patients who are suspicious of CAS.