In this study we investigated the impact of acute coronary syndromes (ACSs) on clinical outcomes in patients with unprotected left main coronary artery (ULMCA) stenosis treated with drug-eluting stents (DESs). In this multicenter, retrospective, observational study we enrolled 1,101 patients with ULMCA stenosis treated with DESs. Six hundred eleven patients presented with ACS and 490 had stable coronary artery disease. ACS was defined as the presence of unstable angina or non-ST-segment elevation myocardial infarction (MI). During 2-year follow-up, the adjusted hazard ratio of cardiac mortality and MI of patients with ACS versus stable patients was 2.42 (95% confidence interval 1.37 to 4.28, p = 0.002). We observed a stepwise risk increase, namely patients with stable coronary disease had the lowest risk, patients with unstable angina an intermediate risk, and patients with non-ST-segment elevation MI the highest risk. The increased risk of cardiac mortality and MI of patients with ACS was concentrated in the first year after DES implantation. In conclusion, patients with ULMCA stenosis and ACS treated with DESs have an increased risk of cardiac mortality and MI during the first year after the intervention compared to stable patients.