Over a third of patients admitted to intensive care develop hypernatremia (raised serum sodium concentration) or hyponatremia (reduced plasma sodium concentration) within a few days of admission. Both hypo- and hypernatremia acquired after admission prolong hospital stay and decrease chance of survival. These are the main findings of a 7-year-long epidemiological study conducted at three intensive care units (ICU) that together provide all critical care to a population of 1.2 million in the Canadian city of Calgary and surrounding area. Between January 2000 and December 2006, 12,744 adults (aged 18 years and over) were admitted to the three medical-surgical ICUs. Of these, 8,142 (64 %) had normal serum sodium (133-145 mmol/L) documented during the first calendar day of admission; this population comprised the study cohort. Of these, 2157 (26 %) developed hypernatremia (defined as serum sodium >145 mmol/L) and 917 (11 %) developed hyponatremia (defined as serum sodium <133 mmol/L). The median time from admission to abnormal sodium was 2 days (interquartile range, 1-5 days for hyponatremia and 1-3 days for hypernatremia). 16 % of hyponatremic patients and 19 % of hypernatremic patients suffered more than one distinct episode of hyponatremia or hypernatremia, respectively. Of all 3,074 patients with sodium disturbance, 196 suffered episodes of both hypo- and hypernatremia during their stay in ICU. Investigators recovered a host of clinical, demographic and outcome data for each of the 8,142 study cohort, for comparison of those with and without sodium disturbance. This data contributes to a better understanding of sodium disturbance in the critically ill. So far as outcome is concerned, statistical analysis of data revealed that length of stay and mortality in the ICU were increased for patients with ICU-acquired hyponatremia and hypernatremia compared to patients whose serum sodium concentration remained within the normal range for duration of their admission.