Objective Tolvaptan was first approved for use for cirrhosis in Japan in September 2013. The aim of the study was to examine the effect of tolvaptan, a vasopressin V2 receptor antagonist, on the prognosis of cirrhosis. Methods The effect of tolvaptan was evaluated in 26 patients with cirrhosis treated at our hospital from September 2013 to April 2015. Results The primary disease was hepatitis C in 20 patients, hepatitis B in 2, nonalcoholic steatohepatitis in 2 and others in 2; and 12 had hepatocellular carcinoma. The Child-Pugh score was 9.7±1.6 and the serum albumin level was 2.53±0.44 g/dL. Body weight decreased from 55.5±11.8 kg before administration to 52.1±14.7 kg after 7 days of tolvaptan treatment. After 7 days, patients with weight loss ≥2 kg (n=16, mean decrease of 4.3±2.3 kg) had significantly lower blood urea nitrogen (24.2±14.4 vs. 36.1±11.4 mg/dL) and serum creatinine (1.1±0.5 vs. 1.5±0.7 mg/dL) levels and decreased urine osmolality 4 h after the administration of tolvaptan (236±96 vs. 364±122 mOsm/kg) compared with patients with weight loss <2 kg (n=10, mean increase of +0.7±2.1 kg) (all p<0.05). The prognosis was significantly better in the group with weight loss ≥2 kg. Conclusion The effect of tolvaptan on the renal function is likely to improve the prognosis of patients with cirrhosis if the drug is started at a stage in which the renal function is maintained.