Background: There is a paucity of data on the incidence of mild-to-moderate heart failure (HF) complicating ST-segment elevation acute myocardial infarction (MI) and its impact on short-term outcomes. Our objective was to determine the incidence, timing, and consequences of mild-to-moderate HF complicating acute MI. Methods: We examined the occurrence of death or death/recurrent MI (re-MI) in patients enrolled in the Global Utilization of Streptokinase and Tissue-Plasminogen Activator for Occluded Coronary Arteries (GUSTO-I), the Global Use of Strategies to Open Occluded Coronary Arteries (GUSTO IIb), the Global Use of Strategies to Open Occluded Coronary Arteries (GUSTO-III), and Assessment of the Safety and Efficacy of a New Thrombolytic (ASSENT-II) trials, which examined different fibrinolytic therapies for MI. We excluded patients who had cardiogenic shock (n = 2994) or unknown HF status at all time points (n = 13,716). Of the remaining 61,041 patients, 17,949 patients (29.4%) had HF, 1566 (8.7%) only at baseline, 10,339 (57.6%) only after admission, and 6044 (33.7%) at baseline and after. Results: The incidence of HF was 32.5% in the United States and 26.9% elsewhere. At 30 days, death and death/re-MI occurred in 2% and 4% of patients without HF and 8% and 12% of patients with HF, respectively (2% and 4% of patients with HF only at baseline, 7% and 13% of patients with HF only after baseline, and 10% and 13% of patients with HF at baseline and later). By use of multivariable analyses, the presence of HF was associated with 1.55 times greater risk of dying at 30 days (95% Cl 1.38-1.74) and 2.15 times greater risk of death/re-MI (95% Cl 1.96-2.36). Conclusion: Mild-to-moderate HF is a frequent and ominous complication of MI, especially when it does not resolve or develops after admission.