BACKGROUND: Although the mainstay of treatment for acute decompensated heart failure (ADHF) is decongestion by diuretic therapy, it is often associated with worsening renal function (WRF). The effect of tolvaptan, a selective V2 receptor antagonist, on WRF in ADHF patients with preserved left ventricular ejection fraction (LVEF) is unknown.Methods and Results:We enrolled 50 consecutive ADHF patients whose LVEF on admission was ≥45%. Patients were randomly assigned to either tolvaptan add-on (n=26) or conventional diuretic therapy (n=24). The primary endpoint was the incidence of WRF, defined as an increase in serum creatinine (Cr) ≥0.3 mg/dL or 50% above baseline within 48 h of randomization. There was no significant difference between the 2 groups in the change in body weight or the total urine volume during 48 h. However, the change in Cr (∆Cr) at 24 and 48 h after randomization and the incidence of WRF (12% vs. 42%, P=0.0236) were significantly lower, and the fractional excretion of urea (FEUN) at 24 and 48 h after randomization was significantly higher in the tolvaptan group. There was an inverse correlation between ∆Cr and FEUN at 48 h after randomization. CONCLUSIONS: Tolvaptan can alleviate congestion with a significantly lower risk of WRF in ADHF patients with preserved LVEF, presumably through maintenance of renal perfusion.