Gulf War syndrome is the name given to a variety of psychological and physical symptoms suffered by veterans of the Gulf War (although it is not clear that any of these symptoms are related to Gulf War service, or to each other). The symptoms have been remarkably wide-ranging, and sometimes somewhat ill-defined, but typically include chronic fatigue, loss of muscle control, diarrhea, migraines, dizziness, memory problems and loss of balance.
Various causes that have been suggested, some more likely than others:
United States Veterans Affairs Secretary Anthony Principi's panel found that more recent studies suggest the veterans' illnesses are neurological and apparently are linked to exposure to neurotoxins such as the nerve gas sarin, the anti-nerve gas drug pyridostigmine bromide and pesticides that affect the nervous system.
"Research studies conducted since the war have consistently indicated that psychiatric illness, combat experience or other deployment-related stressors do not explain Gulf War veterans illnesses in the large majority of ill veterans," the review committee said.
In November, 2004, an independent British inquiry concluded, for the first time, that thousands of UK and US Gulf War veterans were made ill by their service. The report concluded Gulf veterans were twice as likely to suffer from ill health than if they had been deployed elsewhere. It concluded the illnesses suffered were the result of a combination of causes. These included multiple injections of vaccines, the use of organophosphate pesticides to spray tents, low level exposure to nerve gas, and the inhalation of depleted uranium dust. 
The study is the first to suggest a direct link between military service in the Gulf and illnesses suffered by veterans of that war and directly contradicts previous theories which had suggested GWS was not a physical illness, but a response to the stresses of war. Similar syndromes have been seen as an after effect of many conflicts - for example, 'shell shock' after World War I, and post-traumatic stress disorder after the Vietnam War. If this were true, the unfortunate effect is that the psychologically based therapy that might help support those affected in their very real distress, is neglected in favour of a futile search for a non-existent physical cause: and repeated studies where a particular cause is found to be unconnected become regarded as further evidence for a cover-up by the authorities.
Another possibility is that some or all of the symptoms experienced are unrelated to service in the Gulf. A study for the United Kingdom Ministry of Defence found no correlation between service in the Gulf and death from illness.
Although not identifying Gulf War syndrome by name, in June of 2003 the High Court of England and Wales upheld a claim by Shaun Rusling that the depression, eczema, fatigue, nausea and breathing problems that he experienced after returning from the Gulf War were attributed to his military service.
A new British study comparing 24,000 Gulf War veterans to a control group of 18,000 men found that those who had taken part in the Gulf war have lower fertility and are 40 to 50% more likely to be unable to start a pregnancy. Among Gulf war soldiers, failure to conceive was 2.5% vs. 1.7% in the control group, and the rate of miscarriage was 3.4% vs. 2.3%. These differences are small but statistically significant. 
Relationship between Gulf War Syndrome and Multiple Chemical Sensitivity
The symptoms of Gulf War Syndrome are remarkably similar to those associated with Multiple Chemical Sensitivity (MCS) leading some to believe that they are related. This similarity, and the fact that there is no medical test that will diagnose either (because of a lack of coherent symptoms and lack of a clear definition), has led most experts to conclude that neither illness is physical but purely psychosomatic. Others, mostly adherents of alternative medicine, believe that the cause of the illness is physical, although as yet unidentifiable, and that it will eventually be pinpointed, leading to the legitimisation of the illness and the development of successful treatments. This has occurred with asthma, allergies, repetitive strain injury and many other previously 'dubious' complaints. However, asthma, allergies, and RSI have distinct and identifiable symptoms, which the Gulf War Syndrome and MCS have not. Also, the symptoms of asthma and allergies remain the same in double-blind tests, while the symptoms of MCS disappear in test where the subject is unaware of exposure to suspected substances.