Sustained ventricular arrhythmias and heart failure are well-recognized complications after acute myocardial infarction (AMI) and have been associated with worse outcomes and increased mortality. The use of and outcomes associated with acute beta-blocker therapy in patients with AMI complicated by sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) and heart failure were investigated. Of 5,391 patients in the VALIANT Registry, sustained VT/VF occurred in 306 (5.7%), with an in-hospital mortality rate of 20.3%. Multivariable logistic regression identified sustained VT/VF as a major predictor of in-hospital death (relative risk 4.18, 95% confidence interval 2.91 to 5.93). Of those with sustained VT/VF, 55.2% were treated with intravenous or oral beta blockade in the first 24 hours. After adjusting for baseline characteristics, propensity for acute beta-blocker use, and the interaction between Killip classification and beta-blocker therapy, beta-blocker therapy within 24 hours was associated with decreased in-hospital mortality in patients with sustained VT/VF (relative risk 0.28, 95% confidence interval 0.10 to 0.75, p = 0.013) without evidence of worsening heart failure. Patients with sustained VT/VF were less likely to receive beta blockers within 24 hours (p = 0.001). In conclusion, sustained VT/VF was common after AMI. In patients with sustained VT/VF, beta-blocker therapy in the first 24 hours after AMI was associated with decreased early mortality without worsening heart failure. Unfortunately, beta blockers were underused acutely in patients with sustained VT/VF.