This prospective study investigated the common belief that pyrexia is frequently absent in elderly patients with infection. Oral temperature was closely monitored using both a mercury and an electronic thermometer in 150 ill elderly patients (mean age 81 years) of whom 80% were new admissions to this Unit. A scoring system was devised, based on investigation results and excluding temperature, to assess objectively the likelihood of infection. Seventy-one patients (47%) had ‘definite’ infection: 95% were pyrexial. A further eight of the nine patients with probable infection were pyrexial. There were no significant differences in mean temperature or other indices of infection between those who died of their infection and those who survived. Ten per cent of all pyrexias were detected only on the electronic thermometer, not on mercury measurement. In 12% of pyrexial patients, the pyrexia first appeared more than 12 h after temperature measurement started. With effective monitoring, pyrexia is detectable in the vast majority of infected elderly patients.