BACKGROUND AND AIMS: Initial proof suggests that a non-specific intraventricular conduction delay (NIVCD) is a risk factor for mortality. We explored the prognosis of intraventricular conduction delays (IVCD) - right bundle branch block (RBBB), left bundle branch block (LBBB), and the lesser-known NIVCD - in patients with acute coronary syndrome (ACS). METHODS: This is a retrospective registry analysis of 9,749 consecutive ACS patients undergoing coronary angiography and with an ECG recording available for analysis (2007-2018). The primary outcome was cardiac mortality. Mortality and cause of death data (in ICD-10 format) were received from the Finnish national register with no losses to follow-up (until December 31, 2020). The risk associated with IVCDs was analyzed by calculating subdistribution hazard estimates (SDH; deaths due to other causes being considered competing events). RESULTS: The mean age of the population was 68.3 years (SD 11.8). The median follow-up time was 6.1 years (IQR 3.3-9.4), during which 3,156 patients died. Cardiac mortality was overrepresented among IVCD patients: 76.9% for NIVCD (n = 113/147), 67.6% for LBBB (n = 96/142), 55.7% for RBBB (n = 146/262), and 50.1% for patients with no IVCD (n = 1,275/2,545). In an analysis adjusted for age and cardiac comorbidities, the risk of cardiac mortality was significantly higher in all IVCD groups than among patients with no IVCD: SDH 1.37 (1.15-1.64, p < 0.0001) for RBBB, SDH 1.63 (1.31-2.03 p < 0.0001) for LBBB, and SDH 2.68 (2.19-3.27) for NIVCD. After adjusting the analysis with left ventricular ejection fraction, RBBB and NIVCD remained significant risk factors for cardiac mortality. CONCLUSION: RBBB, LBBB, and NIVCD were associated with higher cardiac mortality in ACS patients.