The aim of the study was to assess the clinical significance of complex ventricular arrhythmias (VAs) (sustained ventricular tachycardia (sVT) and ventricular fibrillation (VF)) in patients with ST-segment elevation myocardial infarction (STEMI) depending on timing of arrhythmia. We analyzed 4, 363 consecutive STEMI-patients treated invasively between 2004 and 2014. The median follow-up was 69.6 months (range: 0–139.8 months). The study population was divided into 2 main groups: VA Group encompassed 476 (10.91 %) patients with VAs, whereas 3887 (89.09 %) subjects without VT/VF were included into the Control Group. Among VA-population, pre-reperfusion VA (34.24%; n=163) was the most common arrhythmia, whereas reperfusion-induced, early post-reperfusion and late post-reperfusion VAs were diagnosed in: 103 (21.64 %), 103 (21.64 %) and 107 (22.48 %) patients, respectively. Every type of sVT/VF complicating STEMI portended significantly worse in-hospital prognosis, however a late onset arrhythmia was associated with the highest (over 5-fold) and reperfusion-induced VA with the lowest (less than 3-fold) increase in mortality risk compared to the Control Group. On the contrary, long-term mortality was significantly increased only in subjects with late post-reperfusion and pre-reperfusion VAs compared to VA-free population (43.93% and 36.81%, respectively vs. 22.58%; p<0.001). Apart from cardiogenic shock on admission, late post-reperfusion (HR 3.39) and pre-reperfusion VAs (HR 2.76) were the strongest independent predictors of death in the analyzed population. In conclusion, one in 10 patients with STEMI treated invasively was affected by sVT/VF. The clinical impact of VAs was strongly dependent on timing of arrhythmia.