Context: Hospital-acquired hypernatremia (HAH) is a frequent concern in critical care, which carries high mortality. Aims: To study the risk factors for HAH in settings that practice a preventive protocol. Settings and Design: Two tertiary-care hospitals. Prospective observational study design. Materials and Methods: Patients aged >18 years admitted for an acute medical illness with normal serum sodium and need for intensive care >48 h formed the study population. Details of the basic panel of investigations on admission, daily electrolytes and renal function test, sodium content of all intake, free water intake (oral, enteral and intravenous) and fluid balance every 24 h were recorded. Individuals with serum Na 140-142 meq/l received 500 ml of free water every 24 h, and those with 143-145 meq/l received 1000 ml free water every 24 h. Statistical Analysis Used: Risk factors associated with HAH was analysed by multiple logistic regression. Results: Among 670 study participants, 64 (9.5%) developed HAH. The median duration of hypernatremia was 3 days. A total 60 of 64 participants with HAH had features of renal concentrating defect during hypernatremia. Age >60 years ( P = 0.02), acute kidney injury (AKI) on admission ( P = 0.01), mechanical ventilation ( P = 0.01), need for ionotropes ( P = 0.03), worsening Sequential Organ Failure Assessment (SOFA) score after admission ( P < 0.001), enteral tube feeds ( P = 0.002), negative fluid balance ( P = 0.02) and mannitol use ( P < 0.001) were the risk factors for HAH. Mortality rate was 34.3% among hypernatremic patients. Conclusions: The study suggests that administration of free water to prevent HAH should be more meticulously complied with in patients who are elderly, present with AKI, suffer multi-organ dysfunction, require mechanical ventilation, receive enteral feeds and drugs like mannitol or ionotropes.