Temporary equipment may be brought onto a plant site for a variety of process needs. The risk profile of the facility is altered by use of such temporary equipment, but since the time durations of temporary equipment usage are generally small, usually fewer than 90 days per year, the risk increase is also small and is typically not modeled in a probabilistic risk assessment. This article describes some risk issues with temporary equipment in long-term use, gives a case study of a temporary, diesel-driven air compressor under extended use at the Idaho National Laboratory, and suggests the need to model temporary equipment in the facility risk assessment if the equipment is used more than a few months. (C) 2006 American Institute of Chemical Engineers.