BACKGROUND: Despite initial in-hospital treatment of acute heart failure (HF), some patients experience worsening HF (WHF). There are limited data about the outcomes and characteristics of patients who experience in-hospital WHF. METHODS AND RESULTS: We assessed the characteristics and outcomes of patients with and without WHF in the ASCEND-HF trial. Worsening HF was defined as at least 1 symptom or sign of new, persistent, or WHF requiring additional intravenous inotropic/vasodilator or mechanical therapy during index hospitalization. We assessed the relationship between WHF and 30-day mortality, 30-day mortality or HF hospitalization, and 180-day mortality. We also assessed whether there was a differential association between early (days 1-3) vs late (day ≥4) WHF and outcomes. Of 7,141 patients with acute HF, 354 (5%) experienced WHF. Patients with WHF were more often male and had a history of atrial fibrillation or diabetes, lower blood pressure, and higher creatinine. After risk adjustment, WHF was associated with increased 30-day mortality (odds ratio 13.37, 95% CI 9.85-18.14), 30-day mortality or HF rehospitalization (odds ratio 6.78, 95% CI 5.25-8.76), and 180-day mortality (hazard ratio 3.90, 95% CI 3.14-4.86) (all P values < .0001). There was no evidence of a difference in outcomes between early and late WHF (all P values for comparison ≥ .2). CONCLUSIONS: Worsening HF during index hospitalization was associated with worse 30- and 180-day outcomes. Worsening HF may represent an important patient-centered outcome in acute HF and a focus of future treatments.