The vasopressin type 2 receptor antagonist tolvaptan (TLV) has recently become available to treat congestion in in-hospital patients with heart failure (HF). However, there is no evidence confirming the long-term efficacy of TLV. The Assessment of Quality of life during long-term treatment of TLV in refractory heart failure (AQUA-TLV) study is a multicenter, open-labeled, randomized, controlled clinical trial that will enroll 100 patients from 18 hospitals in Japan. Patients with HF assigned to New York Heart Association class III or IV, who had a previous history of hospitalization due to congestive HF during the past 1 year and ongoing symptomatic congestion with baseline urine osmolality > 350 mOsm/L regardless of being prescribed daily furosemide ≥ 60 mg are randomized to the conventional diuretics group (50 patients) and TLV add-on group (50 patients), and their quality of life will be assessed using the Minnesota Living with Heart Failure Questionnaire after 6 months of treatment. This study is the first multicenter prospective randomized study in Japan to evaluate the long-term clinical effectiveness of TLV compared with conventional treatment in patients with congestive HF (UMIN Clinical Trial Registry Number: UMIN 000009604).