Background:Although diuretic resistance leading to residual congestion is a known predictor of a poorer heart failure (HF) prognosis, better therapeutic strategies for effective and safe decongestion have not been established.Methods and Results:In this study, 81 HF patients with fluid retention (despite taking ≥40 mg/day furosemide (FUR)), with an estimated glomerular filtration rate <45 mL/min/1.73 m2, were randomized into 2 groups and administered either ≤15 mg/day additive tolvaptan (TLV) or ≤40 mg/day increased FUR for 7 days. Changes in urine volume between baseline and mean urine volume during treatment were significantly higher in the TLV than FUR group (P=0.0003). Although there was no significant decrease in body weight or improved signs and symptoms of congestion between the 2 groups, the increase in serum creatinine on Day 7 from baseline was significantly smaller in the TLV than FUR group (P=0.038). Multiple logistic regression analysis revealed that additive TLV (odds ratio 0.157, 95% confidence interval 0.043–0.605, P=0.001) was an independent clinical factor for improved renal function during treatment compared with increased FUR.Conclusions:In HF patients with residual congestion and renal dysfunction refractory to standard therapy, additive TLV increased urine volume without further renal impairment compared with patients who received an increased dose of FUR.