Ascites accompanying a malignancy is often refractory to conventional treatment with saline diuretics, making it difficult to control. We administered a new diuretic, Tolvaptan, to 10 individuals with malignancy and heart failure accompanied by ascites, which was refractory to saline diuretics, and assessed its efficacy and adverse events. We observed a significant reduction in abdominal distension following 2 weeks of Tolvaptan administration. However, we also observed significant increases in serum potassium, urea nitrogen, and creatinine levels, but no serious adverse events. This suggests that Tolvaptan may also be effective as treatment for ascites.