SummaryTolvaptan, a vasopressin type 2 receptor antagonist, does not affect kidney circulation or cause worsening of renal function (WRF) in patients with acute decompensated heart failure (ADHF). Bioelectrical impedance analysis (BIA) can be used to evaluate intravascular volume by calculating the ratio of extracellular water (ECW) to intracellular water (ICW). There have been no reports examining the mechanisms of tolvaptan-induced diuresis using BIA. We investigated whether tolvaptan decreases excess volume while maintaining intravascular volume in ADHF patients.Study patients included 29 ADHF patients (age 48-95, men 69%) diagnosed between April 2013 and May 2016 and who underwent BIA before and after treatment. Fifteen patients were treated with tolvaptan in addition to conventional diuresis therapy (tolvaptan group), and 14 patients were treated with conventional diuresis therapy only (control group). In the control group, the numerical value of serum creatinine (Cre) significantly increased from 0.89 ± 0.22 mg/ dL to 1.07 ± 0.29 mg/dL (P = 0.004), and the ECW/ICW significantly decreased from 0.696 ± 0.036 to 0.673 ± 0.032 (P = 0.004). These values were not significantly different from those obtained for the tolvaptan group. Furthermore, regression analysis showed a negative correlation between ΔCre and ΔECW/ICW, which are the differences between values before and after treatment (ΔCre = -0.002-5.668 × ΔECW/ICW, r 2 = 0.306, P = 0.002). Our findings suggest that WRF is caused by a reduction in intravascular volume and that tolvaptan treatment can decrease the excess volume while maintaining intravascular volume. (Int Heart J 2017; 58: 593-600)