ASHINGTON, Dec. 10 — After years of denying any link between illness and service in the Persian Gulf war, military officials said today that veterans of the conflict were nearly twice as likely as other soldiers to suffer the fatal neurological illness known as Lou Gehrig's disease.

The joint announcement by the Defense and Veterans Affairs Departments was based on the preliminary findings of a study of more than 2.5 million veterans. Officials said they would immediately offer disability and survivor benefits to affected patients and families. Forty cases have been identified so far.

"The hazards of the modern-day battlefield are more than bullet wounds and saber cuts," Veterans Affairs Secretary Anthony J. Principi said in releasing the results of the study. "We have to be conscious of that and act accordingly."

Some scientists said the decision might be premature because the research had not been subjected to the analysis of peer review. An epidemiologic analysis, the study tracks patterns of disease, and does not prove that gulf war service was the cause of the disorder, formally called amyotrophic lateral sclerosis, or A.L.S.

The authors did not offer theories on why gulf war veterans would be at increased risk. Nor did they say what the odds were that the finding occurred by chance. Conducted at a veterans' hospital in Durham, N.C., the study has been submitted for publication in an academic journal. Officials would not identify it.

Veterans' groups, who have long argued that their members are afflicted by unexplained illnesses collectively called Gulf War syndrome, were ecstatic. So were patients' advocates, who said the study might yield important clues about the cause of the disease.

More than 100,000 American service members sent to the region in 1990 and 1991 have reported a range of maladies including fatigue, muscle pain, memory loss and sleep disorders. But studies have found no definitive links, although several have suggested that the ailments may stem from to chemical exposure, stress or prophylactic medicines given to soldiers.

"This may give some insight into an environmental exposure that would have something to do with A.L.S.," Mike Havlicek, president of the ALS Association, which represents the 30,000 Americans with the disease, said about the new study. "We are very supportive of the effort to get to the bottom of this."

A.L.S. is extremely rare, and the risk for soldiers is small. Of nearly 700,000 who fought in the yearlong gulf conflict, the study identified 40 with A.L.S. About half have died. In a comparison group of 1.8 million people who served in the military at the same time but were not in the region, 67 developed A.L.S.

The study found that among the nearly 700,000 soldiers sent to the gulf from August 1990 to July 1991, the risk of contracting A.L.S was 6.7 per million. Among the 1.8 million who were not deployed, the risk was 3.5 per million.

Though the overall risk was twice as high among those deployed, it was not consistent across the services. Air Force veterans were 2.7 times more likely to develop A.L.S. than those not deployed. Army soldiers faced twice the risk. But Navy and Marine Corps veterans did not show rates of disease that were statistically higher than those not deployed.

The epidemiologist who led the research, Dr. Ron Horner, said he and his colleagues drew on military records, appealed to veterans and patients' groups and made other efforts to identify veterans who had received diagnoses of A.L.S. in the 10 years since the war. He said his team had interviewed veterans with A.L.S. and their doctors and studied medical records and death certificates of those who had died.

Pentagon and veterans' officials said that the Durham study might be the largest of its kind and that they were confident of the results. "The risk is elevated, and the risk is statistically significant," said Dr. John R. Feussner, chief of research and development for the Veterans Affairs Department.

He and Secretary Principi promised to pursue research that would help identify the cause of the illnesses. They said they could not wait for a complete scientific review before informing patients of the results.

Dr. Feussner said the results were presented today at meeting of researchers. "Our first order of business," he said, "is to take care of our patients."

The announcement was a startling turnabout for the military, and it is quite likely to renew focus on gulf veterans' health problems.

"It's just a shame it has taken so long," said Representative Christopher Shays, a Connecticut Republican who has held hearings on veterans' health problems. "There has been an incredible reluctance on the part of the Defense Department to acknowledge any gulf war illness. So I consider this a huge announcement."

The Pentagon assistant secretary for health affairs, Dr. William Winkenwerder, described the shift as a result of "an evolving thought process" about the risks of combat.

"The science," said Richard J. Santos, national commander of the American Legion, "is sound and the evidence is clear — U.S. troops were exposed to something in the Persian Gulf that accounts for an increased rate of A.L.S." We applaud the administration for deciding to award just compensation and benefits immediately."

A.L.S., which causes severe muscle wasting, is typically fatal in two to five years. Scientists have identified two genes connected with the A.L.S., which came to public attention when Lou Gehrig of the Yankees contracted it. The inherited version accounts for 15 percent of all cases. The rest are classified as sporadic.

"The fact that we have been able to find a higher incidence in gulf war veterans is a big step forward to, maybe, getting at the cause," said Sharon Hesterlee, chief of research and development for the Muscular Dystrophy Association, which pays for A.L.S. research.

Reports of A.L.S. among young veterans were particularly disturbing, Dr. Hesterlee said, because the disease typically afflicts people 45 and older. Two other studies had failed to show a link to gulf war service.

The $1.3 million study began in March 2000, and the next step, Dr. Feussner said, is for scientists to "look to see if there is any cluster or anything these soldiers have in common that might give us a clue as to what the cause is."