Abstract: A probe assembly for an infrared medical thermometer, which is adapted for insertion into a patient's ear canal and which facilitates a rapid measurement of the patient's body temperature with very high accuracy. The probe assembly includes a plastic outer tube sized to fit snugly in the patient's ear canal, with an elongated heat sink and an infrared sensor located within the tube. A cylindrical shield and inwardly-projecting guard ring ensure that the sensor's hot junction receives radiation only from the ear canal and that the sensor's cold junction is prevented from being heated for at least sufficient time to allow the thermometer to provide an accurate measurement of the patient's body temperature.