Acute heart failure (AHF) emerges either de novo or from worsening of chronic heart failure (CHF). The aim of the present study was to evaluate the association between worsening of CHF and mortality in AHF patients. Out of 152 included AHF patients, 47 (30.9%) were de novo AHF patients and 105 (69%) were AHF patients with worsening of CHF. The proportion dying in hospital (19.0% vs. 4.3%, p = 0.023) and within 3 months after hospitalization (36.6% vs. 6.7%, p < 0.001) was significantly higher in AHF patients with worsening of CHF. Logistic regression analyses also showed a significant positive association of AHF emerging as worsening of CHF with hospital mortality [odds ratio (OR) and 95% confidence interval (CI): 5.29 (1.46-34.10), p = 0.029] and 3-month mortality [8.09 (2.70-35.03), p = 0.001]. While the association with hospital mortality was no longer significant after adjusting for comorbidities and clinical as well as laboratory parameters known to be associated with mortality in heart failure patients, the association with 3-month mortality remained significant. We conclude that compared to de novo AHF, AHF evolved from worsening of CHF is a more severe condition and is associated with increased mortality.