BACKGROUND: Hypernatraemia is associated with morbidity and mortality, particularly in the older person. Last summer, Ireland experienced prolonged periods of excessive heat. The Irish meteorological service defines a heatwave as temperatures exceeding 25°C for five consecutive days. AIM: This study sought to compare the frequency of hypernatraemia (sodium (Na+) >145 mmol/l) observed during a modest heatwave with that during average ambient temperature in the temperate Irish climate. DESIGN: Retrospective cross-sectional analysis with nested case-control study. METHODS: The 10-day period from 24 June to 3 July in 2017 and 2018 were chosen as the control and heatwave periods, respectively. Patients aged >65 with at least one Na+ value recorded on the laboratory information system were included. Local meteorological data, age, gender and Na+ levels were evaluated. RESULTS: Maximum air temperatures were significantly higher during the heatwave period (mean 27°C vs. 16.8°C, P?