The incidence and significance of hypernatremia (raised plasma sodium) acquired during critical illness is the subject of a recently published retrospective study conducted at the combined medical and surgical intensive care unit of a Brisbane hospital where plasma sodium of all patients is regularly monitored (usually 6 hourly) at the point of care using a blood gas analyzer. The study cohort comprised 3317 patients admitted to the unit between 2002 and 2006 with a normal plasma sodium (135-145 mmol/L) at the time of admission. Each of these patients was assigned to one of three groups depending on plasma sodium concentration over the duration of their stay in the unit. This first (hypernatremic) group comprised all those (n=263) whose peak plasma sodium was > 150 mmol/L; the second (intermediate group) comprised all those (n=419) whose peak sodium was in the range 145-150 mmol/L, and the third (normonatremic) group comprised all those (n = 2239) whose plasma sodium remained within the normal range (135-145 mmol/L) for the duration of their admission. The incidence of acquired hypernatremia of the entire cohort was found to be 7.7 %. Mortality rate was higher in the hypernatremic group (35 %) than in both the intermediate group (21.0 %) and the normonatremic group (7.7 %). After controlling for age, admitting disease category and disease severity, hypernatremia was found to be an independent predictor of mortality implying that normalization of serum sodium in patients with hypernatremia and treatments aimed at preventing hypernatremia might improve the chance of surviving critical illness.