This prospective study assessed the accuracy of the infrared tympanic thermometer (ITT) compared to the rectal thermometer (RT) using statistical measures of agreement. In a convenience sample of 100 adult emergency department patients, ear examinations to assess for cerumen or otitis were followed by temperature measurements using the First Temp 2000A thermometer in both ears and the IVAC 2000 rectally. Left and right ITT temperatures showed high correlation and agreement; therefore, only right ITT results are reported. Both the ITT and RT recorded similar mean temperatures, standard deviations, and ranges. The correlation of the ITT and RT and agreement were below the 0.8 level, indicating excellent agreement. The mean temperature difference (RT-ITT) between the two devices was 0.1 +/- 0.7 degrees C; in 10% of patients, the temperature difference was > or = 1 degree C. Among 10 patients identified as febrile by RT (RT > or = 38.5 degrees C), 6 were febrile by ITT. Significant differences occurred between the temperature measurements using the ITT and RT; these devices do not demonstrate excellent agreement.