Background Hypernatraemia is associated with a short-term mortality of 20-60%. Age-related physiological changes predispose patients to hypernatraemia. This study reviewed acutely admitted patients comparing those with community-acquired (CAH) and hospital-acquired hypernatraemia (HAH). Methods A retrospective study of 102 consecutive acute medical in-patients with serum [Na]>145 mmol/L was conducted. Baseline characteristics, clinical presentation, laboratory values, monitoring, management and outcomes were compared between CAH and HAH groups. Results Patients were exclusively older (>69 years). Forty patients (39.2%) had CAH and sixty-two (61.8%) had HAH. Those with CAH were more likely to be NH residents, have dementia and reduced mobility. Most HAH patients had mild hypernatraemia initially (75.8%, n=47), and higher rates of acute kidney injury (27% (n=11) vs 8% (n=3)/p=0.02) were observed. Monitoring was inadequate and no patient had a free water deficit documented. Medication review and intravenous fluid prescribing was similar between groups. The median length of stay of discharged HAH patients was longer (22.5 vs 8 days/p=0.005). Mortality rates were similar (47% (n=29) vs 37% (n=15)/p=0.416). Time from admission to death was higher in HAH patients (16 vs 8 days/p=0.008). Conclusions Both CAH and HAH present similarly, however, older patients with cognitive/physical impairments are at an increased risk. Early identification of high-risk patients and adherence to best practice guidelines is required.