Both renal function and serum sodium level are well-known prognostic markers in heart failure (HF) patients. We investigated the prognostic value of the renal impairment (RI) stratified by the serum sodium level in systolic HF patients.The Clinical Characteristics and Outcomes in Relation with Serum Sodium Level in Asian Patients Hospitalized for Heart Failure (the COAST) Study enrolled hospitalized systolic HF patients (ejection fraction<45%) in South Korea, Taiwan, and China. Twelve-month mortality was stratified according to the renal function and serum sodium level.Of 1462 enrolled patients, 716 patients (49%) had RI (GFR<60 mL/min/1.73 m), and they had higher 12-month mortality than those without RI (22.8% vs. 10.9%, P<0.001). Furthermore, 676 patients (46%) had low sodium level defined as Na<median, that was, 139 mmol/L. The mortality rate was lowest in patients with normal renal function and high sodium level (7.4%), but highest in those with RI and low sodium level (26.1%) (P<0.001). Patients with both RI and low sodium level had a 3.8-times increased hazard for 12-month mortality (HR 3.80, 95% CI 2.06-7.05), whereas the low sodium level (HR, 2.95; 95% CI, 1.51-5.75) and RI (HR 3.08; 95% CI, 1.63-5.82) had similar hazard, suggesting that they might be equivalent risk factors.In hospitalized Asian HF-patients both RI and low sodium level are independent risk factors. Patients with both RI and low serum sodium level are at the highest risk and may require meticulous medical care.