A hand-held, battery-powered, electronic instrument designed to measure a patient's body temperature. It may comprise an electronic unit with an attached probe or be a single unit (shaped like an ordinary hand-held capillary thermometer) that detects and converts the changes in temperature into variations of some electrical characteristic, e.g., resistance or voltage. These variations of the electrical characteristics are processed in the electronic circuits and in turn displayed, for a short period, as temperature readings. Thereafter the display will automatically turn off or go into standby mode. This is a reusable device.