OBJECTIVES: To evaluate the cost-effectiveness of tolvaptan for hypervolemic or euvolemic hyponatremia in South Korea. METHODS: A decision tree was constructed to assess the clinical and economic impact over 30 days from restricted societal perspective. A competitor was supportive care. The result was presented as the incremental cost per QALY gained. We supposed that patients would move to 3 different serum sodium levels; normonatremia (>135mEq/L), mild hyponatremia (130-135mEq/L), marked hyponatremia(<130 mEq/L) after 4 days treatment. Each level had three states; discharge, continuing hospitalization and death. According to serum sodium level, Patients had difference their length of hospital stay. Uncertainty was explored with deterministic sensitivity analysis. RESULTS: The analysis showed that cost of tolvaptan was 1,358,370 KRW and supportive care was 1,396,092 KRW. The use of tolvaptan reduced 37,722 KRW for supportive care. QALYs were 0.047203, 0.042146 (tolvaptan, supportive care, respectively). Tolvaptan had 0.00506 QALYs higher. Cost-effectiveness analysis represented tolvaptan was dominant. The deterministic sensitivity analysis for uncertain parameters demonstrated that this analysis results were robust. CONCLUSIONS: Tolvaptan was cost-effective for hyponatremia in comparison with supportive care. The first medicine as oral vasopressin receptor antagonist, tolvaptan could increase patients’ quality of life for hyponatremia in Korean population.