OBJECTIVES: Tolvaptan is a selective vasopressin V2-receptor antagonist indicated for the treatment of adult patients with hyponatraemia (HN) secondary to syndrome of inappropriate antidiuretic hormone secretion (SIADH). To date there have been no published economic evaluations assessing the cost effectiveness of tolvaptan in this indication.  The aim of this study was to evaluate the cost effectiveness of tolvaptan versus no active treatment (NAT) from a Swedish societal perspective. METHODS:  The economic evaluation considers a hypothetical population of individuals with HN secondary to SIADH who have either failed to respond to fluid restriction or for whom the use of fluid restriction is not suitable. The analysis considers three clinically relevant patient populations within the SIADH indication: ‘all SIADH’, small-cell lung cancer (SCLC) and pneumonia.  A discrete event simulation was developed to model the progression of individuals through multiple inpatient admissions over a 30 day time horizon (180 days in the SCLC scenario).  Key sources of evidence included randomised controlled trials (SALT I & II) and observational data sources.  Unit costs were collected from publicly available sources. Utility values were obtained from mapping the SF-12 scores from the SALT I & II trials to EQ-5D. The primary outcome of the analysis was the incremental cost-effectiveness ratio (ICER) expressed as a cost per quality-adjusted life-year (QALY).   RESULTS: In the ‘all SIADH’ population tolvaptan was associated with reduced costs (SEK 5,778) and increased QALYs (0.0019) versus NAT and was therefore dominant. In the SCLC and pneumonia subgroups tolvaptan was also associated with reduced costs and QALY improvements. The results were most sensitive to the duration of tolvaptan treatment and the assumptions around duration of hospitalisation.   CONCLUSIONS:  In all populations considered (all SIADH, SCLC and pneumonia) tolvaptan was dominant compared to NAT being associated with reduced costs and increased QALYs.