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Background: Hyponatremia in cancer patients is often caused by the syndrome of inappropriate antidiuretic hormone secretion (SIADH). The aim of this observational multicenter study was to analyze the medical and economic implications of SIADH in this setting. Methods: This study included 90 oncological patients from 28 Italian institutions that developed SIADH between January 2010 and September 2015. Data on clinical–pathological characteristics, anticancer therapies, hyponatremia, and related treatments were statistically analyzed. Results: The majority were lung cancer patients (73%) with metastatic disease at the onset of hyponatremia (83%). A total of 76 patients (84%) were hospitalized because of SIADH and less than half (41%) received tolvaptan for SIADH treatment. The duration of hospitalization was significantly longer in patients who did not receive tolvaptan and in those who do not reach sodium normalization during hospitalization. Patients who experienced a second episode of hyponatremia following tolvaptan dose modification/discontinuation presented a significantly lower serum sodium value at the time of hospitalization and minimum sodium value during hospitalization compared with patients who had not experienced another episode. The severity of hyponatremia, defined as minimum sodium value during hospitalization with a cut-off value of 110 mmol/l, and not obtaining sodium correction during hospitalization significantly correlated with overall survival rate. Conclusions: Hyponatremia due to SIADH could result in longer hospitalization and in a decreased overall survival when not adequately treated, and tolvaptan represents an effective treatment with a potential effect of both improving overall survival and decreasing duration of hospitalization.