Comparison of rectal, axillary, and tympanic membrane temperatures in infants and young children. STUDY OBJECTIVE: To evaluate the reliability of a tympanic membrane thermometer in detecting fever in young children presenting to the emergency department. SETTING: Pediatric emergency department in an urban teaching hospital, DESIGN/MEASUREMENT/PARTICIPANTS: Temperature measurements were obtained sequentially at three body sites in children less than 3 years old presenting to the pediatric ED. Axillary and rectal temperatures were obtained with an electronic thermistor probe (Diatek 500), and tympanic membrane temperatures were obtained with a noncontact, infrared sensing device (First TEMP). Patients were stratified by age, ear canal patency, presence of otitis media, and rectal temperature. RESULTS: Of 224 patients enrolled, 87 (39%) were febrile. Overall correlation of axillary and tympanic membrane measurements to rectal for all strata was .75 (P = .001) and .81 (P = .001), respectively. Sensitivity in detecting fever for axillary and tympanic membrane sites was .48 and .55, respectively. Otitis media and ear patency did not influence correlation of tympanic membrane measurements. Low tympanic membrane temperature sensitivity may be a result of probe configuration. CONCLUSION: Tympanic membrane and axillary temperatures should be viewed with caution in children less than 3 years old as neither can detect fever reliably. 