Objective: The blood urea nitrogen-to-creatinine (BUN/creatinine) ratio has been proposed as a useful parameter in acute heart failure (AHF), but data on the normal range and the added value of the ratio compared with its separate components in patients with AHF are lacking. The aim of this study is to define the normal range of BUN/creatinine ratio and to investigate its clinical significance in patients with AHF. Methods: In 4484 subjects from the general population without cardiovascular comorbidities, we calculated age-specific and sex-specific normal values of the BUN/creatinine ratio, deriving a higher and lower than normal range of BUN/creatinine ratio (exceeding the 95% prediction intervals). Association of abnormal range to prognosis was tested in 2033 patients with AHF for the outcome of all-cause death through 180 days, death or cardiovascular or renal rehospitalisation through 60 days and heart failure (HF) rehospitalisation within 60 days. Results: In a cohort of patients with AHF, 482 (24.6%) and 28 (1.4%) patients with HF were classified into higher and lower than normal range groups, respectively. In Cox regression analysis, higher than normal range of BUN/creatinine ratio group was an independent predictor for all-cause death (HR: 1.86, 95% CI 1.29 to 2.66) and death or cardiovascular or renal rehospitalisation (HR: 1.37, 95% CI 1.03 to 1.82), but not for HF rehospitalisation (HR: 1.23, 95% CI 0.81 to 1.86) after adjustment for other prognostic factors including both creatinine and BUN. Conclusions: In patients with AHF, BUN/creatinine higher than age-specific and sex-specific normal range is associated with worse prognosis independently from both creatinine and BUN.