On October 17, 2003, four male career fire fighters were slightly injured while participating in a live fire training evolution inside a propane-fueled mobile flashover training simulator. Multiple burn evolutions had been conducted over a two and one-half day period without incident prior to the explosion. Minutes after the four-man crew advanced a hoseline into the mobile training trailer, an explosion occurred. The crew members were knocked down by the force of the explosion and became momentarily disoriented, but were able to exit the trailer under their own power. A fifth male fire fighter stationed on the outside of the trailer was also slightly injured when he was hit by an exit door that he was monitoring which was forced open by the blast. NIOSH investigators concluded that, to minimize the risk of similar occurrences, fire departments should: 1. follow manufacturer's recommendations for set-up and operation of training simulators; 2. consider using a carbon monoxide monitor to ensure carbon monoxide does not build up to a dangerous level inside the training simulator; 3. ensure that Standard Operating Guidelines (SOGs) specific to live-fire training are developed and followed; and, 4. consider having an ambulance on-site during live-fire training exercises. Additionally, training simulator manufacturers should: 1. provide ventilation systems within training simulators that ensure a complete air change is accomplished between burn evolutions; and, 2. consider using programmable logic controllers (PLC) and computerized electronic data monitoring systems to avoid the possibility of improper operation. Additionally, the National Fire Protection Association (NFPA) should consider developing a new standard covering mobile live-fire training simulators.