Abstract: A fluid warming device for warming blood and other fluids prior to introduction into a patient. The counterflow fluid bags utilize a flat counter flow pathway which provides high flow performance and ensuring uniform, gradual and energy efficient blood warming. The warming device contains a pair of opposed, identical heater elements. The guide rails of the bag align the counter flow passageway for the fluid so that the bag is spaced and located precisely between the heater elements. The heater is provided with a receptive slot down the middle, in a horizontal plane, with two different sized guide slots, one on each side of the receptive slot. A pair of guide rails, integrated into the sides of the bag, are parallel to one another providing sufficient rigidity for easy insertion of the bag, and being sized to allow easy insertion in only the correct orientation. The blood warmer preferably incorporates a microprocessor for precise control of the electric current provided to the heater.