Background: As a relatively high number of ST-segment elevation myocardial infarction (STEMI) patients develop major adverse cardiovascular events (MACE) following percutaneous coronary intervention (PCI), our aim was to determine the significance, and possible predictive value of QRS complex width and ST-segment elevation. Methods: Our patient sample included 200 PCI-treated STEMI patients, which were divided into two groups, based on the duration of symptoms: (I) less than 6 hours, and (II) 6-12 hours. For every patient, ECG was performed at 6 different time points. Long-term follow-up lasted 6 years from the STEMI onset. Statistical analysis was performed. Results: The mean age was 60.6±11.39 years, and 142 were male. Wider QRS complex, more pronounced ST-segment elevation, and more frequently absent ST-segment resolution were more common in the second group. Presentation of MACE was more frequent in this group. An increase of pre-PCI QRS complex width by 1 msec increases the risk of 6-year MACE by 3%, and an increase in pre-PCI ST-segment elevation by 1mm increases the risk of 6-year MACE by 3.26 times. Conclusion: To the best of our understanding, this study represents a pioneer research that utilized these ECG elements in the context of predicting a 6-year MACE. A wider QRS complex and a more pronounced ST-segment elevation were associated with longer total ischemic time. More research is needed to confirm these findings.