cluster of illnesses in veterans of the Persian Gulf War (1990–91) characterized not by any definable medical condition or diagnostic test but by variable and nonspecific symptoms such as fatigue, anxiety, muscle and joint pains, headaches, memory loss, and posttraumatic stress reactions. Gulf War syndrome is believed to be caused by exposure to a class of chemicals known as anticholinesterases (anticholinesterase). These chemicals are extremely toxic and have been associated with neuropsychological dysfunction and overall poor health in agricultural workers who routinely apply anticholinesterase-based pesticides (pesticide) to their crops. Anticholinesterases to which Gulf War veterans could have been exposed include nerve toxins such as sarin, the prophylactic anti-nerve-toxin drug carbamate pyridostigmine bromide, and organophosphate- or carbamate-containing insecticides (insecticide). Some people carry a gene variant that increases their susceptibility to anticholinesterase toxicity.

Gulf War syndrome does not appear to be fatal but can be associated with considerable distress and disability. As a group the Gulf War veterans seem to report higher frequencies of these symptoms than do veterans of other wars; in one study up to 17 percent of British veterans believed they had Gulf War syndrome. There is general agreement that Gulf War syndrome is not a distinct disease. At first many experts categorized it with other so-called functional somatic syndromes, such as chronic fatigue syndrome and fibromyalgia, which presumably represent aberrant responses to various stressors such as anxiety and infection. The symptoms of some veterans with Gulf War syndrome resemble those of chronic fatigue syndrome. Some veterans who believe they have Gulf War syndrome have been diagnosed as having other definable conditions.