Background: Very few data have been reported for ST-segment elevation myocardial infarction (STEMI) caused by unprotected left main coronary artery (ULMCA) occlusion, and very little is known about the results of this subgroup of patients who underwent primary percutaneous coronary intervention (PCI). The aim of this study was to determine the clinical features and outcomes of patients with STEMI who underwent primary PCI for acute ULMCA occlusion. Methods: From January 2000 to February 2014, 372 patients with STEMI caused by ULMCA acute occlusion (ULMCA-STEMI) who underwent primary PCI at one of two centers were enrolled. The 230 patients with non-ST-segment elevation MI (NSTEMI) caused by ULMCA lesion (ULMCA-NSTEMI) who underwent emergency PCI were designated the control group. The main indexes were the major adverse cardiac events (MACEs) in-hospital, at 1 month, and at 1 year. Results: Compared to the NSTEMI patients, the patients with STEMI had significantly higher rates of Killip class≥Ⅲ (21.2％ vs.3.5％,x2 = 36.253,P< 0.001) and cardiac arrest (8.3％ vs.3.5％,x2 = 5.529, P = 0.019). For both groups, the proportions of one-year cardiac death in the patients with a post-procedure thrombolysis in myocardial infarction (TIMI) flow grade<3 were significantly higher than those in the patients with a TIMI flow grade of 3 (STEMI group: 51.7％ [15/29] vs.4.1％ [14/343], P < 0.001; NSTEMI group: 33.3％ [3/9] vs.13.6％ [3/221], P = 0.001; respectively]. Landmark analysis showed that the patients in STEMI group were associated with higher risks of MACE (16.7％ vs.9.1％, P = 0.009) and cardiac death (5.4％ vs.1.3％,P=0.011) compared with NSTEMI patients at 1 month. Meanwhile, in patients with ULMCA, the landmark analysis for incidences of MACE and cardiac death was similar between the STEMI and NSTEMI (all P = 0.72) in the intervals of 1-12 months. However, patients who were diagnosed with STEMI or NSTEMI had no significant difference in reinfarction (all P> 0.05) and TVR (all P>0.05) in the intervals of 0-1 month as well as 1 month to 1 year. The results of Cox regression analysis showed that the differences in the independent predictors for MACE included the variables of Killip class≥ Ⅲ and intra-aortic balloon pump support for the STEMI patients and the variables of previous MI, ULMCA distal bifurcation, and 2-stent for distal ULMCA lesions for the NSTEMI patients. Conclusions: Compared to the NSTEMI patients, the patients with STEMI and ULMCA lesions still remain at a much higher risk for adverse events at 1 year, especially on 1 month. If a successful PCI procedure is performed, the 1 -year outcomes in those patients might improve.