OBJECTIVE To examine whether, in clinical practice, the infrared tympanic thermometer shows temperature readings similar to those obtained with the rectal digital thermometer, so that the former can replace the latter. METHODS Prospective comparative study. METHODS Academic Hospital, Nijmegen, the Netherlands. METHODS In 104 patients admitted to the department of medicine, body temperature was measured by both methods within approximately ten minutes. This was done on two successive days. The measurements were then analysed by plotting the difference between two measurements against their mean. Then the limits of agreement, which are the mean of the differences between the two measurements plus and minus 2 standard deviations, were determined. With both thermometers also duplicate measurements were made to study the repeatability. RESULTS The mean difference between the 2 methods in the first measurement was 0.15 degree C (SD: 0.56), in the second measurement it was 0.07 degree C (0.52). The limits of agreement were 1.27 degrees C and -0.97 degree C for the first comparisons and 1.13 degrees C and -0.99 degree C for the second comparisons. In the duplicate measurements, the mean difference between the first and the second measurement was 0.02 degree C (0.19) in the rectal measurement, and 0.09 degree C (0.23) in the tympanic measurement. The patients found the tympanic measurements significantly less painful and unpleasant than the rectal measurement. The mean time needed for the tympanic measurements (8 s) was ten times less than for the rectal measurements (79 s). CONCLUSIONS The results of this study show good agreement between the infrared tympanic thermometer and the rectal digital thermometer so that they may be regarded as interchangeable. The patients had a clear preference for the tympanic thermometer, which also took less time.