AbstractObjectivesTo determine all‐cause in‐hospital mortality associated with severe hypernatraemia and the causes, comorbidities, time to treatment, discharge destination and postdischarge mortality.DesignRetrospective observational cohort study.PatientsSevere hypernatraemia, (sodium concentration ≥ 155 mmol/L), at any time during a tertiary hospital admission in Melbourne, Australia, 1 January 2019 to 31 December 2019 (pre‐COVID19).MeasurementsDeaths, Charlson Comorbidity Index (CCI), hypernatraemia causes, time to treatment, discharge destination.ResultsOne hundred and one inpatients: 64 community‐acquired, 37 hospital‐acquired. In‐hospital mortality was 38%, but cumulative mortality was 65% by 1 month after discharge, with only a minor further increase at 6 and 12 months. After adjusting for peak sodium concentration, the community acquired group had significantly reduced odds of in‐hospital mortality (odds ratio 0.15, 95% confidence interval [0.04−0.54], p = .003). Iatrogenic factors were present in 57% (21/37) of the hospital‐acquired group. Only 55% of all cases received active sodium directed treatment. Time to start treatment did not affect outcomes. High levels of comorbidity were present, median CCI (IQR) was 6 (5−8) in the community and 5 (4−7) in the hospital group. Dementia prevalence was higher in the community group, 66% (42/64) versus 19% (7/37) (p = .001). Infection was the most common precipitant with 52% (33/64) in the community and 32% (12/37) in the hospital group. Of the survivors, 32% who had been living independently required residential care after discharge.ConclusionsMortality was high and loss of independence in survivors common. To potentially improve outcomes, hypernatraemia‐specific guidelines should be formulated and efforts made to reduce system and iatrogenic factors.