The government "has not conceded that vaccines cause autism," said Linda Renzi, the lawyer representing federal officials, who have consistently maintained that childhood shots are safe.

However, parents and advocates for autistic children see the case as a victory that may help certain others. Although the science on this is very limited, the girl's disorder may be more common in autistic children than in healthy ones.

"It's a beginning," said Kevin Conway, a Boston lawyer representing more than 1,200 families with vaccine injury claims. "Each case is going to have to be proved on its individual merits. But it shows to me that the government has conceded that it's biologically plausible for a vaccine to cause these injuries. They've never done it before."

A lawyer for the 9-year-old girl has scheduled a news conference in Atlanta on Thursday. Her parents have declined to comment in the meantime because the case is not final and the payment amount has not been set.

Nearly 5,000 families are seeking compensation for autism or other developmental disabilities they blame on vaccines and a mercury-based preservative, thimerosal. It once was commonly used to prevent bacterial contamination but since 2001 has been used only in certain flu shots. Some cases contend that the cumulative effect of many shots given at once may have caused injuries.

The cases are before a special "vaccine court" that doles out cash from a fund Congress set up to pay people injured by vaccines and to protect makers from damages as a way to help ensure an adequate vaccine supply. The burden of proof is lighter than in a traditional court, and is based on a preponderance of evidence. Since the fund started in 1988, it has paid roughly 950 claims - none for autism.

Studies repeatedly have discounted any link between thimerosal and autism, but legal challenges continue. The issue even cropped up in the presidential campaign, with Republican John McCain asserting on Friday that "there's strong evidence" autism is connected to the preservative.

The girl has a disorder involving her mitochondria, the energy factories of cells. The disorder - which can be present at birth from an inherited gene or acquired later in life - impairs cells' ability to use nutrients, and often causes problems in brain functioning. It can lead to delays in walking and talking.

Federal officials say the law bars them from discussing the case or releasing documents without the family's permission. However, The Associated Press obtained a copy of the concession by U.S. Department of Health and Human Services officials.

According to the document, five vaccines the girl received on one day in 2000 aggravated her mitochondrial condition, predisposing her to metabolic problems that manifested as worsening brain function "with features of autism spectrum disorder." In the 1990s, the definition of autism was expanded to take in a group of milder, related conditions, which are known as autism spectrum disorders.

The document does not address whether it was the thimerosal - or something else entirely in the vaccines - that was at fault.

A Portuguese study suggested that 7 percent of autistic children might also have the mitochondrial disorder, versus one in 5,000 people - or 0.02 percent - in the general population, said Dr. Marvin Natowicz, a Cleveland Clinic geneticist.

"Even if they're off by a factor of seven" and only 1 percent are afflicted, "it's still a striking statistic," he said.

Others said they doubt the Georgia case will have much effect.

"No link between mitochondrial disorders and autism spectrum disorder has been made in mainstream medicine," said Dr. Michael Pichichero of the University of Rochester in Rochester, N.Y., who has consulted for the government on vaccines and has received speaking fees from vaccine makers.

A decision is expected this spring on the first test case for a larger group of autism-vaccine claims, which are being heard in the U.S. Court of Federal Claims.

Reported cases of autism have been rising in the U.S., even after thimerosal was removed from most childhood vaccines. However, some experts believe the rise is due to an expansion of the definition of autism and related conditions, and a desire to diagnose children so they qualify for special services and aid.

Associated Press writer Kristen Gelineau in Richmond, Va., and medical writer Mike Stobbe in Atlanta contributed to this story.