Dr. Drew A. Helmer, director of the Department of Veterans Affairs’ War-Related Illness and Injury Study Center in New Jersey, called the Georgetown studies “very preliminary” but also “a very important step forward.”

But Dr. John Bailar, an emeritus professor at the University of Chicago who led a group that studied gulf war illness in 1996, said the new study did not provide enough data to determine whether the veterans’ symptoms were linked to their deployments to Kuwait, or something entirely different.

“I am not questioning whether a substantial proportion of veterans of Desert Storm have symptoms related to their service,” Dr. Bailar said in an e-mail. “I am questioning whether those symptoms have any cause other than the stress of war itself.”

Studies by the Department of Veterans Affairs have estimated that as many as 250,000 of the nearly 700,000 service members who served in the Middle East in 1990 and 1991 have reported symptoms of gulf war illness, which is also known as chronic multisymptom illness.

Gulf war illness has been the source of much frustration and dispute practically since veterans first reported symptoms in the 1990s. Many veterans say that their complaints were initially dismissed as psychological. Many also believe that their problems are the result of exposure to nerve agents, pesticides, herbicides and other chemicals, but that the government has been slow, or unwilling, to pinpoint causes.

Even some government researchers have made that case. At a Congressional hearing in March, Dr. Steven S. Coughlin, an epidemiologist who once worked for the Department of Veterans Affairs, asserted that the department had systematically played down the neurological basis of gulf war illness. At the same hearing, a member of an advisory panel to the department said the agency still seemed guided by the view that symptoms of gulf war illness were stress-induced.

“This is a throwback to early speculation from the 1990s that there was no problem, or that veterans just had random, disconnected symptoms,” testified Dr. Lea Steele, a Baylor University epidemiologist who was a member of the Research Advisory Committee on Gulf War Veterans’ Illnesses.