Background Left main coronary artery (LMCA) stenosis has been recognized as a risk factor for early death among patients undergoing coronary artery bypass grafting (CABG).This study aimed to assess if LMCA lesions pose an additional risk of early or mid-term mortality and/or a major adverse cardiac and cerebrovascular event (MACCE) after off-pump coronary artery bypass grafting (OPCABG),compared with non-left main coronary artery stenosis (non-mainstem disease).Methods From January 1,2009 to December 31,2010,4869 patients had a primary isolated OPCABG procedure at Beijing Anzhen Hospital.According to the pathology of LMCA lesions,they were retrospectively classified as a non-mainstem disease group (n=3933) or a LMCA group (n=936).Propensity scores were used to match the two groups,patients from the non-mainstem disease group (n=831) were also randomly selected to match patients from the LMCA group (n=831).Freedom from MACCE in the two groups was calculated using the Kaplan-Meier method.Results The difference in the mortality and the rate of MACCE during the first 30 days between the non-mainstem disease group and the LMCA group did not reach statistical significance (P=0.429,P=0.127 respectively).With a mean follow-up of (12.8±7.5) months and a cumulative follow-up of 1769.6 patient-years,the difference in the freedom from MACCEs between the two groups,calculated through Kaplan-Meier method,did not reach statistical significance (P=0.831).Conclusion Analysis of a high volume of OPCABG procedures proved that LMCA lesions do not pose additional early and mid-term risk to OPCABG.Therefore,a LMCA lesion is as safe as non-mainstem disease lesion during the OPCABG procedure.