Aim: SIADH may be related to several causes (pulmonary disorders, CNS disturbances, drugs and chemotherapeutic agents), but in the majority of cases it is caused by malignancies. This observational study aimed to analyze treatments and outcome of SIADH in cancer pts. Methods: This study includes 69 consecutive cancer pts who experienced SIADH between 2010 and 2014 in 22 Italian Cancer Centers. Data on clinico-pathology, anticancer and SIADH treatments were recorded and statistically analized. Results: M/F ratio was 47/22, median age was 67 years (range 37-83). Primary tumor was lung cancer in 50 pts (SCLC in 39), while it was GI cancer in 8 pts. Fifty-eight pts were hospitalized due to SIADH, HN at admission was ≤130 mEq/l in 94.3%. Median duration of hospitalization was 13 days (range 3-90). 31 pts received tolvaptan for SIADH treatment (group A); other treatments included hypertonic saline solutions, diuretics, fluids restriction (group B). Group A included a significant higher N° of lung cancer pts (77.4% vs 68.4%) with metastatic disease (80.6% vs 76.3%). Moreover pts in group A had more severe HN at admission: serum sodium was ≤130 mEq/l in 96.7% pts and <120 mEq/l in 48% of pts, while in group B it was ≤130 mEq/l in 66.6% of pts and <120 mEq/l in 42.9%. Median sodium at the beginning of tolvaptan was 120 mEq/l (range 110-130), in group B it was 117 mEq/l (range 109-132). In group A 13 pts started tolvaptan 15 mg/daily, 4 started with 30 mg and 14 with 7.5 mg. No toxicity due to tolvaptan was observed in 25 pts, 6 experienced dry mouth. In 19 pts a dose change was required: in 4 it was increased to 30 mg/daily, while in the remaining 15 it was decreased to 7.5 mg every other day. HN improvement with tolvaptan was observed in all cases. Lenght of hospitalization was significantly longer in pts not receiving tolvaptan (15 days, range 4-100, vs 12 days, range 3-53 in pts receiving tolvaptan vs 8 days, range 1-41 in pts receiving tolvaptan since start of tolvaptan, p = 0.002). Severity of HN and not obtaining its correction correlated with lenght of stay and with overall survival (p < 0.05). Conclusions: To our knowledge this is the largest study analyzing SIADH in cancer pts to demonstrate that HN due to SIADH may result in a prolongation of hospitalization and in a worse overall survival when not adequately corrected and that tolvaptan represents an effective treatment potentially improving both. Disclosure: All authors have declared no conflicts of interest.