Gulf War syndrome has no formal medical definition. But that is just the start of the problem.

Since it first emerged in the complaints of Army reservists from Indiana in 1992, the term has come to mean a group of symptoms that commonly include fatigue, joint pains, skin rash, diarrhea, sleeplessness, sleep disturbances, difficulty concentrating and mood instability. How big the problem is and what explains it remain issues of maddening uncertainty and rancorous disagreement, despite a decade of research.

Researchers have established with near certainty that Gulf War veterans have not died or been hospitalized at higher rates than other veterans who served at the same time. Nor have they had more children with birth defects. But determining how many of the 697,000 men and women who served in the Gulf subsequently became chronically ill has been much harder.

More than 100,000 people enrolled in registries established by the Department of Veterans Affairs and the Department of Defense in the years immediately after the war's end. Muscle and joint pain were the most frequently cited symptoms, reported by 40 percent of people in the VA registry and by 52 percent of those in the Defense Department registry. Fatigue and headache were reported by one-fourth to nearly one-half of the people, with dozens of other complaints less common.

Because those who registered were a self-selected group, however, they gave an unreliable estimate for the whole population of Gulf veterans. A survey begun by VA epidemiologists in 1995 provided a better one.

The researchers selected 15,000 Gulf War veterans and 15,000 veterans who served elsewhere at the same time and asked them to fill out a long questionnaire about 42 symptoms and numerous other things. Gulf veterans complained of all the symptoms at higher rates, and 23 percent described their health as fair or poor.

If the VA sample truly represents Gulf War veterans as a whole, then as many as 160,000 men and women may consider themselves to be in less than optimal health since the war. Many theories have been offered over the years about the cause of the problems, including the effects of smoke, dust, vaccines, tropical infections, depleted-uranium ammunition, minute quantities of chemical weapons agents, the drug pyridostigmine bromide and pesticides. But 14 expert panels, including a presidential commission that met for two years, rejected all those explanations.

Nearly all the panels pointed to psychological stress -- and in particular the threat of chemical and biological attack -- as a likely cause of some illnesses. The stress theory, however, is controversial, and greatly resented by many veterans.

After every American conflict dating at least to the Civil War, physicians have noted a syndrome of vague physical complaints in large numbers of veterans -- presumably arising from the stress of war. Gulf War syndrome, many think, is this war's version of that phenomenon.

Evidence for unusual levels of stress-related psychiatric illness in Gulf veterans is inconsistent. The VA study found post-traumatic stress syndrome in 10 percent of Gulf veterans, compared with 4 percent of non-Gulf veterans. Some other studies, however, have found much lower rates.

Today, the scientific consensus is that Gulf War syndrome is not a unique entity and has no specific cause. Instead, it is a catch-all label for problems with myriad causes.

At the severe end of the spectrum are people with serious degenerative ailments (such as Lou Gehrig's disease or early-onset Alzheimer's) for which the causes are unknown but are believed by many current sufferers to have arisen from their Gulf War service. In the middle are people with psychiatric disorders that have physical manifestations. At the mild end -- accounting for most of the complaints -- are people whose symptoms are little different from those experienced daily by the general population, but have been magnified by attention and attributed to the Gulf War.

Not everyone agrees. A few scientists continue to seek a single cause, or cluster of war-time exposures, to account for Gulf War syndrome.

-- David Brown