BACKGROUND: Young patients (<50years) exhibit specific characteristics in chronic heart failure (HF), but their phenotype in acute heart failure (AHF) is not well described. METHODS AND RESULTS: 2033 patients of the PROTECT trial were divided into two groups: young patients (≤50years) and older patients (>50years). Biomarkers from different pathophysiological domains were available in 1266 patients. Patients were compared with regard to clinical characteristics, biomarker profiles, and in-hospital (worsening renal function [WRF] and decongestion) and post-discharge (180-day survival) outcome. Young patients (n=121) were mostly men, had fewer comorbidities with better renal function, and more often had a reduced ejection fraction. At admission, young patients were more likely to have jugular venous distension, but less rales and dyspnea compared with older patients. During hospitalization, young patients received higher loop diuretic doses and were decongested earlier than older patients. WRF occurred less frequently in young patients (5.9% vs. 13.3%, p=0.020) and they were more often discharged alive. At 180days, the mortality of young patients was lower than that of the older patients (9.9% vs. 18.1, p=0.021). Biomarker levels indicative of inflammation and renal damage were lower in the young, although they exhibited higher BNP levels than older patients. CONCLUSIONS: Despite use of higher diuretic doses, young patients with AHF less often developed WRF during hospitalization and had better outcomes than older patients. Differences in biomarker levels between the age groups suggest distinct underlying pathophysiologies. https://clinicaltrials.gov numbers NCT00328692 and NCT00354458.