This study evaluated the clinical impact of Q-wave presence on ECG at presentation of patients with ST-segment elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI).From April 2005 to September 2009, 184 consecutive STEMI patients who underwent primary PCI within 12 hours of chest pain onset were retrospectively evaluated. Patients were grouped according to the presence (Q positive, n = 109) or absence (Q negative, n = 75) of Q waves on initial ECG at emergency room presentation. Major adverse cardiac events (MACE) and stent thrombosis (ST) were evaluated for 2 years. Risk factors for MACE and left ventricular (LV) remodeling by echocardiography were also evaluated.Baseline characteristics, including reperfusion time and infarct location, were similar between the groups. The MACE rate at 2 years was higher in the Q-positive group (32.1%) than in the Q-negative group (13.3%, P = 0.005). Independent risk factors for MACE were the presence of Q-wave (P = 0.008, Odds ratio 3.139) and no-reflow phenomenon (P = 0.016, Odds ratio, 2.819). LV remodeling was more frequent in the Q-positive group (47.9%) than in the Q-negative (24.5%, P = 0.009) group. Initial Q-wave presence (P = 0.048, Odds ratio 2.380) and anterior wall MI (P = 0.009, Odds ratio, 3.425) were independent risk factors for LV remodeling.The presence of Q waves in ECG of patients presenting with STEMI undergoing primary PCI provides an independent prognostic marker of clinical outcomes and left ventricular remodeling.