Background The association of sustained ventricular tachycardia/fibrillation (VT/VF) with mortality in patients undergoing primary percutaneous coronary intervention (PCI) may vary with baseline patient risk and may be associated with higher mortality in patients who have high-risk baseline features but not in the low-risk patient cohort undergoing this procedure. Methods Among the 5,259 ST Elevation Myocardial Infarction (STEMI) patients presenting for primary PCI from the APEX AMI trial, we evaluated the association of VT/VF with outcomes according to underlying risk for 90-day mortality estimated using baseline variables and with a Cox regression model. Results Ventricular tachycardia/fibrillation occurred in 3.6% (63/1,736), 4.9% (87/1,788), and 8.1% (141/1,735) of patients in the low-, intermediate-, and high-tertiles of 90-day predicted death, respectively. Ninety-day death was between 3.2- and 4.8-fold higher in patients with VT/VF compared with those without it in the 3 risk groups (low risk 1.6% vs 0.5%, intermediate risk 5.7% vs1.2%, high risk 33.6% vs 7.7%). Both early (during cardiac catheterization) and late VT/VF (after cardiac catheterization) were associated with high risk of death regardless of baseline risk category. Conclusions The incidence of VT/VF and mortality increased as patients' baseline risk increased, and VT/VF remained an important prognostic marker for the increased risk of clinical adverse events and 90-day mortality irrespective of underlying baseline risk in patients undergoing primary PCI. Thus, even in otherwise low-risk patients, occurrence of VT/VF helps to further identify higher risk cohort that may warrant closer monitoring.