Purpose: Serum sodium derangement is the most common electrolyte disturbance amongst patients admitted to intensive care. This study aims to validate the association between dysnatraemia and serum sodium fluctuation with mortality in surgical intensive care patients. Method: We performed a retrospective analysis of the Medical Information Mart for Intensive Care (MIMIC-II) database. Dysnatraemia was defined as a sodium concentration outside physiologic range (135???145 mmol/L) and subjects were categorized by severity of dysnatraemia and sodium fluctuation. Univariate and multivariable logistic regression was used to test for associations between sodium fluctuations and mortality. Results: We identified 8600 subjects, 39% female with a median age of 66 years for analysis. Subjects with dysnatraemia were more likely to be dead at 28-days (17% vs 7%; p