['Hypernatremia is common in the intensive care unit (ICU). We assessed the prevalence of hypernatremia and its impact on mortality and ICU length of stay (LOS).', 'Retrospective analysis.', 'All patients admitted to a medical ICU of a university hospital during a 35-month observation period.', 'Hypernatremia (serum sodium > 149 mmol/L) after admission to the ICU.', 'Main outcomes were 28-day hospital mortality and ICU LOS. Demographic factors, main diagnosis, and severity of illness. Cox proportional hazards regression models were used for data analysis.', 'Of 981 patients, 90 (9%) had hypernatremia, on admission to the ICU in 21 (2%) and developed during the ICU stay in 69 patients (7%). Of these 981 patients, 235 (24%) died; LOS was 8 +/- 9 (SD) days. Mortality rates were 39% and 43% in patients with hypernatremia on admission or that developed after admission compared with 24% in patients without hypernatremia (P < 0.01). LOS was 20 +/- 16 days in patients with hypernatremia compared with 8 +/- 10 days in patients without hypernatremia (P < 0.001). In multivariable analysis, hypernatremia was an independent risk factor for mortality (relative risk, 2.1; 95% confidence interval, 1.4 to 3.3).'] Is hypernatremia in the critically ill an independent risk factor for mortality?