Objective To investigate the efficacy and safety of vasopressin receptor antagonist tolvaptan for treating dilutional hyponatremia casused by decompensated liver cirrhosis.Methods Ninety-six subjects with decompensated liver cirrhosis complicated by dilutional hyponatremia were divided into test group (n =56) and control group (n =40) by double blind method.Test group were treated with a single dose of tolvaptan 15 mg orally in addition to routine therapy,while control group were treated with routine therapy plus placebo.The changes in serum sodium concentration,liver functions,ascites,and edema of lower extremities between the two groups were observed.Measurement data were compared by t test,and categorical data were compared by chisquare test.Results On day 7,36 (64.3％) patients in test group and 9 (22.5％) patients in control group reached normal serum sodium concentrations (x2 =5.241,P＜0.01).All the patients in test group and only 3 patients in control group had 24 hours' total urine volume above 3500 mL.Difference of 24 hours' total urine volume during 7-day treatment between the two groups was statistically significant (t=20.899,P＜0.01).Thirty-nine patients in test group and 15 patients in control group had reduced ascites (x2 =4.260,P=0.039),but improvement of edema in lower extremities of both groups was comparable (12 cases in test group and 7 cases in control group; x2 =0.227,P=0.634).Serum alanine aminotransferase (ALT),total bilirubin (TBil),potassium and creatinine levels of test group were improved after treatment,but were not significantly different from either baseline levels (t=1.509,0.783,1.107,1.237; both P＞0.05) or those of the control group (t=1.712,1.635,1.121,0.873; both P＞0.05).Conclusions Single dose of tolvaptan (15 mg) exerts a strong diuretic effect and is able to greatly increase serum sodium concentration,thus has distinct therapeutic effect for patients with decompensated liver cirrhosis complicated by dilutional hyponatremia.