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Yet despite all evidence to the contrary, the number of parents who blame thimerosal for their children's autism has only increased. And in recent months, these parents have used their numbers, their passion and their organizing skills to become a potent national force. The issue has become one of the most fractious and divisive in pediatric medicine.

"This is like nothing I've ever seen before," Dr. Melinda Wharton, deputy director of the National Immunization Program, told a gathering of immunization officials in Washington in March. "It's an era where it appears that science isn't enough."

Parents have filed more than 4,800 lawsuits - 200 from February to April alone - pushed for state and federal legislation banning thimerosal and taken out full-page advertisements in major newspapers. They have also gained the support of politicians, including Senator Joseph I. Lieberman, Democrat of Connecticut, and Representatives Dan Burton, Republican of Indiana, and Dave Weldon, Republican of Florida. And Robert F. Kennedy Jr. wrote an article in the June 16 issue of Rolling Stone magazine arguing that most studies of the issue are flawed and that public health officials are conspiring with drug makers to cover up the damage caused by thimerosal.

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In 2000, a group of parents joined together to found SafeMinds, one of several organizations that argue that thimerosal is that environmental culprit. Their cause has been championed by politicians like Mr. Burton.

"My grandson received nine shots in one day, seven of which contained thimerosal, which is 50 percent mercury as you know, and he became autistic a short time later," he said in an interview.

In a series of House hearings held from 2000 through 2004, Mr. Burton called the leading experts who assert that vaccines cause autism to testify. They included a chemistry professor at the University of Kentucky who says that dental fillings cause or exacerbate autism and other diseases and a doctor from Baton Rouge, La., who says that God spoke to her through an 87-year-old priest and told her that vaccines caused autism.

Also testifying were Dr. Mark Geier and his son, David Geier, the experts whose work is most frequently cited by parents.

Trying to Build a Case

Dr. Geier has called the use of thimerosal in vaccines the world's "greatest catastrophe that's ever happened, regardless of cause."

In 2003, spurred by parents' demands, the C.D.C. asked the Institute of Medicine, an arm of the National Academy of Sciences and the nation's most prestigious medical advisory group, to review the evidence on thimerosal and autism.

In a report last year, a panel convened by the institute dismissed the Geiers' work as having such serious flaws that their studies were "uninterpretable." Some of the Geiers' mathematical formulas, the committee found, "provided no information," and the Geiers used basic scientific terms like "attributable risk" incorrectly.

In contrast, the committee found five studies that examined hundreds of thousands of health records of children in the United States, Britain, Denmark and Sweden to be persuasive.

A study by the World Health Organization, for example, examined the health records of 109,863 children born in Britain from 1988 to 1997 and found that children who had received the most thimerosal in vaccines had the lowest incidence of developmental problems like autism.

Another study examined the records of 467,450 Danish children born from 1990 to 1996. It found that after 1992, when the country's only thimerosal-containing vaccine was replaced by one free of the preservative, autism rates rose rather than fell.

In one of the most comprehensive studies, a 2003 report by C.D.C. scientists examined the medical records of more than 125,000 children born in the United States from 1991 to 1999. It found no difference in autism rates among children exposed to various amounts of thimerosal.

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