A portable intravenous fluid warming system wherein the temperature monitoring and control is accomplished utilizing four monolithic IC thermosensors. The sensors operate in a serial multi-stage fashion with the first sensor functioning so as to provide a ramp or build up to the next stage thus smoothly and gently increasing the temperature of the fluid and reducing the dynamic range required for the final heater stage, thus ensuring a more accurate and consistent output temperature. Power FET's provide heater drive current from an unregulated DC input source. The sensor is coupled to a microprocessor that is factory preprogrammed with predetermined set points. The microprocessor will continuously determine the temperature so as to allow determination of when the desired temperature is reached and will adjust power to the FET as necessary to hold the temperature at the correct point without exceeding the temperature. Interfacing is provided to an optional external digital readout unit for monitoring. The addition of an in-line thermocircuit breaker will provide an independent over-temperature shutdown failsafe.