We report a retrospective study of hypernatraemia (serum sodium concentration greater than 150 mmol/l) in an adult in-patient population of a health district during one year. The incidence was 0.3% with at least 60% of cases developing after hospital admission, mainly in elderly patients. Dehydration appeared to be the major cause, with the use of diuretics, depressed conscious level or febrile illness implicated in a majority. Most patients had more than one contributory factor and iatrogenic causes were common. Associated illnesses were often severe and the in-hospital mortality was high (54%) regardless of age. Hypernatraemia in hospitalized patients should be largely avoidable and there is a need for greater awareness of the importance of active maintenance of hydration in susceptible patients.