Tolvaptan (TLV), a vasopressin type 2 receptor antagonist, has been demonstrated to be effective in patients with decompensated heart failure (HF) refractory to incremental doses of diuretics, but the responsiveness has not always been predictable. We have recently proposed that urine osmolality (U-OSM) is a valuable parameter for the prediction of responses to TLV, because U-OSM reflects the activity of the collecting ducts, where TLV plays its unique role. Acute kidney injury (AKI) is often associated with severe tubular dysfunction, including the collecting ducts, and in such cases a response to TLV may not be expected. We here experienced a patient with HF and AKI in whom TLV was not effective during AKI. We also observed recovery of responsiveness to TLV along with remission of AKI as well as increased U-OSM later on. We believe that this is the first report on the reversibility of the TLV response in relation to U-OSM.