Objective: 15 oncologic patients who developed SIAD were enrolled. Patients receiving TVP belonged to group A, whereas hyponatremic patients treated by hypertonic saline solutions and fluid restriction formed Group B. Methods: Sodium level at admission was 120.4±2.9 mmol/l. Group A achieved the correction of serum sodium, defined at 135 mmol/l, after 3.7±2.8 days. In group B, patients had a similar trend for sodium increase (ΔNa:7.4±4.2 mmol/l/24h), but the target levels were obtained more slowly, after 5.2±3.1 days (p: 0.01), than group A. The hospital stay was longer in group B than TVP patients (17.6±4.7 vs 11.2±3.4 days; p< 0.01). Group B had a higher incidence of re-hospitalization than group A. In group A, 37% of patients had hyponatremic relapses, notwithstanding the progressive increase of doses from 7.5 until 60 mg per day of TVP, revealing a complete lack of response to TVP. These patients revealed growth of tumor mass or new metastatic lesions. Results: TVP improved hyponatremia more efficiently and stably than hypertonic solutions and fluid restrictions, with a high rate of chemotherapeutical cycles concluded. Positive consequences were obtained about the hospital stay, relapse rate of hyponatremia, and re-hospitalization. Conclusions: Objective: Tolvaptan (TVP), a vasopressin receptor antagonist, represents a therapeutic option in the syndrome of inappropriate anti-diuresis (SIAD), inducing a net increase in free water excretion. This study aimed to evaluate the effect of TVP on hyponatremia in oncologic patients. We assessed the incidence of re-hospitalization due to hyponatremia.    Methods: 15 oncologic patients who developed SIAD were enrolled. Patients receiving TVP belonged to group A, whereas hyponatremic patients treated by hypertonic saline solutions and fluid restriction formed Group B. Results: Sodium level at admission was 120.4±2.9 mmol/l. Group A achieved the correction of serum sodium, defined at 135 mmol/l, after 3.7±2.8 days. In group B, patients had a similar trend for sodium increase (ΔNa:7.4±4.2 mmol/l/24h), but the target levels were obtained more slowly, after 5.2±3.1 days (p: 0.01), than group A. The hospital stay was longer in group B than TVP patients (17.6±4.7 vs 11.2±3.4 days; p< 0.01). Group B had a higher incidence of re-hospitalization than group A. In group A, 37% of patients had hyponatremic relapses, notwithstanding the progressive increase of doses from 7.5 until 60 mg per day of TVP, revealing a complete lack of response to TVP. These patients revealed growth of tumor mass or new metastatic lesions. Conclusions: TVP improved hyponatremia more efficiently and stably than hypertonic solutions and fluid restrictions, with a high rate of chemotherapeutical cycles concluded. Positive consequences were obtained about the hospital stay, relapse rate of hyponatremia, and re-hospitalization.