Objectives: Determine the incidence and predisposing factors of acquired-hypernatraemia in the intensive care units (ICU) and its impact on the outcome. Design: Observational cross-sectional study with prospective analysis. Setting: Surgical, medical and trauma intensive care units of National Hospital of Sri Lanka. Study Population: 174 consecutive patients were included in this study. Definition: Hypernatraemia was defined as serum sodium concentration > 145 mmol/l. Results: 74 patients (42.5%) developed hypernatraemia after admission to the intensive care units. Incidence in medical, surgical and trauma ICUs were 47%, 48% and 31% respectively. Significantly lower incidence was reported in patients with trauma compared to the patients from the other two ICUs. High APACHE II (Acute Physiology and Chronic Health Evaluation) score, low GCS (Glasgow Coma Scale), organ dysfunction, transfusion of blood and blood products were associated with an increased incidence of hypernatraemia. Hypernatraemic patients had received significantly greater volume of intravenous fluids exceeding their daily fluid requirement. Compared to normonatraemic patients, hypernatraemic patients demonstrated a longer length of stay (LOS) in the ICU (mean 4.8 days versus 11 days, p< 0.001) and a higher ICU-mortality rate (15% versus 43%, p<0.001). Conclusions: Severity of the illness, inappropriate intravenous fluid therapy and blood transfusions contribute to the incidence of hypernatraemia in intensive care units. It is associated with increased risk of ICU-mortality and longer length of stay in the ICU.