The present invention is directed to a fluid warming cassette (10). The cassette has two polymeric films (32,34) and at least two guide rails (30a,30b). A difference between this fluid warming cassette and those of the prior art is that this fluid warming cassette has a tongue section (20) thereon. This tongue section assists the user insert the cassette into a warming unit's aperture (102). That aperture is relatively narrow because the heating elements of the warming unit must be sufficiently close to the cassette to alter the temperature of the fluid in the cassette to a desired temperature. In addition, the user can confirm if the cassette is properly inserted into the warming device's aperture (102) by seeing that the tongue portion is sticking out of the other end of the warming device's aperture (120). <；IMAGE>；
