This cross-sectional study enrolled 95 elderly Emergency Department patients aged 60 years or more to determine if rectal temperatures identify fevers more often than oral or tympanic temperatures when the chief complaint suggests an infection. A fever was defined as a temperature greater than 38 degrees C (100.4 degrees F). Discordance was defined as any patient with an oral or tympanic temperature of 38 degrees C or less but manifesting a rectal temperature greater than 38 degrees C and 0.5 degrees C (1 degrees F) greater than the oral or tympanic temperature. Rectal thermometry identified a fever in 14 of 95 (14.7%) patients who were afebrile orally and in 11 of 90 (12.2%) patients who were afebrile tympanically. Five of 90 (5.6%) patients were febrile rectally but were afebrile by both oral and tympanic thermometry. Thus, rectal thermometry identified fevers missed orally and tympanically in elderly patients whose presentation suggested infection. To identify these febrile patients with possible infection, clinicians must be attentive to elderly patients' vague clinical presentation.