OBJECTIVES: Syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH) is a common cause of hyponatremia contributing to 30-50% of hyponatremia cases. Little is known of the influence of SIADH on healthcare resource utilization. This study assessed the effect of SIADH on inpatient total and intensive care unit (ICU) cost and length of stay (LOS), the likelihood of ICU admission, and 30-, 90-, and 180-day readmission. METHODS: The Premier hospital database was utilized to identify US hospital inpatients discharged between January 1, 2007 and June 30, 2009. Hyponatremic/SIADH patients were identified using primary or secondary ICD-9 codes (n=430,731) and were matched to a control group (n=430,731) using exact matching on age, gender, provider region and 3M™ APR-DRG assignment. Matching was further refined using propensity scores based on additional patient and hospital covariates. Due to the contribution of congestive heart failure and cirrhosis on hyponatremia development, these patients were excluded from the analysis. The final analytic sample contained 65,973 SIADH patients and 407,874 non-hyponatremia/SIADH patients. Cost was analyzed using gamma regression, LOS with negative binomial regression. ICU admission and hospital readmission were analyzed using multivariate logistic regression. RESULTS: In contrast to non-SIADH patients, patients with SIADH had significantly higher total inpatient cost (55.53%,CI=52.53-58.60;p<0.0001), ICU cost (38.07%;CI=33.18-43.15;p<.0001), total LOS (45.11%,CI=43.20-47.03;p<0.0001), and ICU LOS (42.72%,CI=38.36-47.23;p<0.0001). SIADH patients were significantly more likely to be admitted to the ICU (OR=2.131;p<.0001), and readmitted at 30- (OR=1.399;p<0.0001), 90- (OR=1.495;p<0.0001), and 180-days (OR=1.459;p<0.0001) in comparison with non-SIADH patients. CONCLUSIONS: The presence of SIADH in hospitalized patients is significantly associated with increased total and ICU cost and LOS, likelihood of ICU admission, and likelihood of readmission. Words = 297