SUMMARYBackground: Heart failure (HF) with hypoalbuminemia is associated with poor response to conventional therapy. We investigated whether tolvaptan, a potent aquaretic agent, might be of benefit in HF patients with hypoalbuminemia. Methods: We prospectively enrolled 40 patients hospitalized for HF. Patients received conventional therapy including loop diuretics. We subsequently added tolvaptan in the range of 3.75-15 mg daily and it was discontinued after improvement of HF symptoms. We compared clinical and laboratory data in HF patients with and without hypoalbuminemia (defined as serum albumin <3.0 g/ dL). Results: Tolvaptan was administered in 18 HF patients with hypoalbuminemia (Group A) and 22 HF patients without hypoalbuminemia (Group B). The mean serum albumin was 2.63 AE 0.27 and 3.46 AE 0.25 g/dL, respectively. The average urine output on tolvaptan increased significantly in both groups (1644.4 AE 797.6-3011.6 AE 1453.8 mL/day, P = 0.004; 1459 AE 612.7-2112.2 AE 724.5 mL/day, P = 0.008; respectively). In addition, we observed higher urine output on therapy in Group A than in Group B (P = 0.015). There was a moderate negative correlation between serum albumin and average urine output on tolvaptan (r = À0.42, P = 0.007). Conclusions: The addition of tolvaptan to low dose loop diuretics might be an effective strategy for treatment of HF patients with hypoalbuminemia.