Purpose: The clinical benefit of spironolactone in heart failure (HF) is well described in several large randomized clinical trials. Current guidelines recommend spironolactone administration in all HF patients (pts) with estimated glomerular filtration rate > 30 ml/min/1.73m2. However, the clinical benefits have not been studied in HF pts with severe renal dysfunction (eGFR< 45 ml/min/1.73m2) until now. We investigated the relationship between spironolactone use and all cause mortality in pts with acute heart failure (AHF). Methods: We retrospectively analyzed data from the Korean Heart Failure (KorHF) Registry which hospitalized pts with treatment for AHF. AHF pts (n=1,035) with severe renal dysfunction were included in this study (735 males, 70±13 years old, 444 ischemic origin, 37.2±15.1% left ventricular ejection fraction). Results: Mean eGFR, creatinine and median BUN level were 30.0±11.2 ml/min/1.73m2, 2.33±1.76 mg/dL and 34.8±18.8 mg/dL. During follow-up period (median 407, IQR 108–905 days), 435 (18.5%) pts died and 347 (33.5%) patients had spironolactone therapy. In Kaplan-Meier survival analysis, all cause mortality in spironolactone group was significantly lower compared to non-spironolactone group (18.7% vs. 25.3%, log rank p=0.013). This clinical benefit of spironolactone use was blunted in pts chronic kidney disease (CKD) stage 5 (eGFR<30 ml/min/1.73m2, n=441, 29.9% vs. 28.4%, p=0.877), compatible with the recommendations of current guideline. In subgroup analysis (30≤eGFR<45 ml/min/1.73m2, n=594), aldosterone effect was preserved in low BUN (<25mg/dL, 8.9% vs. 20.1%, p=0.013, n=329) but attenuated in high BUN group (≥25mg/dL, 18.9% vs. 25.3%, p=0.143, n=265). Conclusion: We found that clinical benefit of spironolactone was preserved in pts with severe renal dysfunction, excluding CKD stage 5, especially in low BUN (<25mg/dL) group. The spironolactone therapy based on eGFR and BUN level may have clinical benefit in HF pts with severe renal dysfunction.