Although elevated blood urea nitrogen (BUN)-to-creatinine (BUN/Cr) ratio at hospital admission has been reported to be associated with poor short-term prognosis, its association to long-term mortality in patients with acute decompensated heart failure (ADHF) remains to be elucidated. Moreover, an additive prognostic value to preexisting renal markers including creatinine and BUN has not been well described. A cohort of 557 consecutive ADHF patients admitted to the cardiac intensive care unit was studied. All cohorts were divided into high and low BUN/Cr ratios according to the median value of BUN/Cr ratio at admission. Association between admission BUN/Cr ratio and long-term all-cause mortality was assessed. There were 145 deaths (27%) observed during the follow-up period of 1.9 years in median. Patients with high BUN/Cr ratio showed with higher mortality compared to low BUN/Cr ratio (log-rank: P = 0.006). In the multivariable analysis, patients with high BUN/Cr ratio at admission were associated with high mortality independently from other covariates including BUN and creatinine (HR 1.81, 95% CI 1.16–2.80, P = 0.009). In patients with ADHF, there is a relationship between admission BUN-to-creatinine ratio and long-term mortality.