Changes in QRS duration and pattern are regarded to reflect severe ischemia in acute coronary syndromes (ACS), and ventricular conduction blocks (VCBs) are recognized high-risk markers in both ACS and acute heart failure. Our aim was to evaluate the prevalence, temporal evolution, association with clinical and angiographic parameters, and impact on mortality of VCBs in ACS-related cardiogenic shock (CS). Data of 199 patients with ACS-related CS from a prospective multinational cohort were evaluated with electrocardiogram data from baseline and day 3. VCBs including left or right bundle branch block, right bundle branch block and hemiblock, isolated hemiblocks, and unspecified intraventricular conduction delay were assessed. Fifty percent of patients had a VCB at baseline; these patients were older, had poorer left ventricular function and had more often left main disease compared with those without VCB. One-year mortality was over 2-fold in patients with VCB compared with those without VCB (68% vs 32%, p<0.001). All types of VCBs at baseline were associated with increased mortality, and the predictive value of a VCB was independent of baseline variables and coronary angiography findings. Interestingly, 37% of the VCBs were transient, i.e., disappeared before day 3. However, 1-year mortality was much higher in these patients (69%) compared to patients with persistent (38%) or no VCB (15%, p<0.001). Indeed, a transient VCB was a strong independent predictor of 1-year mortality. In conclusion, our findings propose that any VCB in baseline electrocardiogram, even if transient, identifies very early patients at particularly high mortality risk in ACS-related CS.