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Background Previous studies have found an increase in mortality among patients with worsening degrees of hyponatraemia. There is limited evidence on the impact of dysnatraemia on outcomes such as length of stay (LOS) and intensive care unit (ICU) admission. Aims To investigate the relationship between admission sodium levels at a quaternary referral hospital and outcomes, including mortality, LOS and admission to ICU. Methods All patients admitted to hospital over a period of 1 year who had a blood sodium level measured on admission were included. Admission sodium levels and related blood results were obtained. Data linkage was performed. Analyses for associations between admission sodium level and primary hospital outcomes (in‐hospital mortality) and secondary hospital outcomes (LOS and admission to ICU) were performed. Case notes of patients with severe hyponatraemia were also reviewed, with a focus on the use of hypertonic saline. Results A total of 6447 patient admissions had an admission blood sodium level performed. Mean age was 55.8 ± 20.8 years. 49.3% were male. Serum sodium ranged from 110 mmol/L to 175 mmol/L. The primary outcome of in‐hospital mortality occurred in 190 patients. An abnormal sodium level (Na <135 mmol/L or >145 mmol/L) was an independent predictor of in‐hospital mortality, and an independent predictor of the combined outcome of in‐hospital mortality and admission to ICU. Hypertonic saline was infrequently used in patients with severe hyponatraemia. Its use did not worsen outcomes. Conclusions An abnormal serum sodium level is a strong marker of poor outcome in acute hospital illness.