Background: Prevalent diabetes at the time of heart failure (HF) diagnosis is associated with a higher risk of death, but the incidence and prognostic importance of new-onset diabetes in patients with established HF remains unknown. Methods: Patients with a first hospitalization for HF in the period 2003-2014 were included and stratified according to history of diabetes. Annual incidence rates of new-onset diabetes were calculated and time-dependent multivariable Cox regression models were used to compare the risk of death in patients with prevalent and new-onset diabetes with patients without diabetes as reference. The model was adjusted for age, sex, duration of HF, educational level and comorbidity. Covariates were continuously updated throughout follow-up. Results: A total of 104,522 HF patients were included in the study, of which 21,216 (19%) patients had diabetes at baseline, and 8164 (10%) developed new-onset diabetes during a mean follow-up of 3.9 years. Patients with new-onset diabetes and prevalent diabetes were slightly younger than patients without diabetes (70 vs. 74 and 77, respectively), more likely to be men (62% vs. 60% and 54%), and had more comorbidities expect for ischemic heart disease, hypertension and chronic kidney disease which were more prevalent among patients with prevalent diabetes. Incidence rates of new-onset diabetes increased from around 2 per 100 person-years in the first years following HF hospitalization up to 3 per 100 person-years after 5 years of follow-up. A total of 61,424 (59%) patients died during the study period with event rates per 100 person-years of 21.5 for new-onset diabetes, 17.9 for prevalent diabetes and 13.9 for patients without diabetes. Compared to patients without diabetes, new-onset diabetes was associated with a higher risk of death (adjusted HR 1.47; 95% CI 1.42-1.52) and prevalent diabetes was associated with an intermediate risk (HR 1.19; 95% CI, 1.16-1.21). Conclusion: Following the first HF hospitalization, the incidence of new-onset diabetes was around 2% per year, rising to 3% after 5 years of follow-up. New-onset diabetes was associated with an increased risk of death, compared to HF patients with prevalent diabetes (intermediate risk) and HF patients without diabetes.