The elderly, the very young, and the sick are known to be adversely affected by high environmental temperatures. In a retrospective open case-note review of 872 patients in a large institution during a hot summer we identified characteristics in the elderly that increase the risk of marginal hyperthermia. Women were more likely to be affected than men (25.6% vs 16.9%). We found an age-related increase in marginal hyperthermia, 15.7% of those below 60 years developed a hyperthermia compared to 18.9% in those between 70-79 years (non-significant), 28.3% in those between 80-89 years (p = 0.01) and 50% in those above 90 years (p < 0.01). There was also a direct relationship between the incidence of hyperthermia and the ambient temperature (29% in the warmer wards, compared to 17.2% in cooler ones; p < 0.01) and with the level of dependence (42.3% of the bedridden group, p < 0.01, and 20.4% of the semi-dependent, p < 0.01, compared to 11.1% of the mobile group). These factors were more significant as predictors of risk than the diagnosis. Identifying high risk patients early and taking appropriate measures to avoid hyperthermia and dehydration is important to try to decrease mortality during heatwaves.