Abstract: A disposable cap for a body temperature detector includes a body having a viewing end and a retaining end. The retaining end includes an inward protrusion that expands over a wider portion of an end of the detector and contracts after the retaining end has passed over the wider portion to snugly secure the cap on the detector. The cap further includes a flange with an aperture therethrough adjacent the viewing end to permit a radiation sensor of the detector to view a target surface. Preferably, the cap is sufficiently large so as to not be insertable into an ear of a human. The cap is formed from a sheet of material, preferably by thermoforming, from a material such as polypropylene, polyethylene, polystyrene, or other similar material which has relatively low hardness and low thermal conductivity properties. The cap has a generally uniform thickness of about 0.020 inch.