OBJECTIVES: To compare risks of hypernatraemia on admission to hospital in persons who were with those who were not identified as care home residents and evaluate the association of hypernatraemia with in-hospital mortality. DESIGN: Retrospective observational study. SETTING: A National Health Service Trust in London. PARTICIPANTS: A total of 21,610 patients aged over 65 years whose first admission to the Trust was between 1 January 2011 and 31 December 2013. MAIN OUTCOME MEASURES: Hypernatraemia on admission (plasma Na > 145 mmol/L) and in-hospital death. RESULTS: Patients admitted from care homes had 10-fold higher prevalence of hypernatraemia than those from their own homes (12.0% versus 1.3%, respectively; odds ratio [OR]: 10.5, 95% confidence interval [CI]: 8.43-13.0). Of those with hypernatraemia, nine in 10 cases were associated with nursing home ECOHOST residency (attributable fraction exposure: 90.5%), and the population attributable fraction of hypernatraemia on admission associated with care homes was 36.0%. After correcting for age, gender, mode of admission and dementia, care home residents were significantly more likely to be admitted with hypernatraemia than were own-home residents (adjusted odds ratio [AOR]: 5.32, 95% CI: 3.85-7.37). Compared with own-home residents, care home residents were also at about a two-fold higher risk of in-hospital mortality compared with non-care home residents (AOR: 1.97, 95% CI: 1.59-2.45). Consistent with evidence that hypernatraemia is implicated in higher mortality, the association of nursing homes with in-hospital mortality was attenuated after adjustment for it (AOR: 1.61, 95% CI: 1.26-2.06). CONCLUSIONS: Patients admitted to hospital from care homes are commonly dehydrated on admission and, as a result, appear to experience significantly greater risks of in-hospital mortality.