Ear thermometers are becoming popular as a method for measuring deep body (core) temperature.To determine the variability of a single user's tympanic membrane (ear) temperature measurements.Forty-two, afebrile, healthy children, and 20 febrile children with acute burns.In afebrile children measurements made in both ears (and within just a few minutes of each other) differed by as much as 0.6 degree C. Operator measurement error, sw of three consecutive measurements, in the same ear, was 0.13 degree C. In the group of febrile, burned children, core temperature was measured hourly at a number of sites (ear, rectum, axilla, bladder). A peak in core temperature occurred approximately 10-12 hours after the burn. Measurement error was calculated in 14 febrile, burned children with a peak temperature in excess of 38 degrees C. For the left ear, measurement error was 0.19 degree C and for the right ear, 0.11 degree C. In the febrile children agreement between the ears was poor. The limits of agreement were 0.4 degree C to -0.8 degree C. It was not possible to predict the occasions when the temperature differences between the ears would be large or small.The measurement error of one recording from the next is probably acceptable at about 0.1 to 0.2 degree C. To limit the variations in temperature of one ear to the other, measurements should be restricted to one of the ears whenever possible and the same ear used throughout the temperature monitoring period. Nurses and parents should take more than one temperature reading from the same ear whenever possible.