A major limitation in acute heart failure (AHF) research has been the lack of an outcome measure paralleling re-infarction in acute coronary syndromes. The aim of the present study was to assess the time course, prognostic factors and outcome of early worsening heart failure (WHF) in patients admitted for AHF to a community hospital. All AHF admissions between December 2003 and March 2004 in a regional medical center were recorded. Patients were followed through admission and for 6 months after discharge. Early WHF was defined as WHF occurring during the initial 7 days from admission. Study endpoints were cardiovascular mortality and WHF (defined as worsening or persistent signs or symptoms of AHF requiring rescue therapy or hospital readmission). Early WHF rate was 29% and was associated with markers of AHF severity on presentation (higher troponin I, lower oxygen saturation, need for mechanical ventilation, intravascular diuretics and inotropes). Early WHF was a powerful predictor of death during 6 months of follow up, with an age-adjusted hazard ratio of 3.3 (95% confidence interval 1.7-6.3). Early WHF is a common adverse event in patients admitted with AHF, and is associated with AHF severity and excessive 6-month mortality. WHF should be considered as a clinically important endpoint in AHF studies.