Previously, large scientific studies have found no association between autism and vaccines containing thimerosal.

But many parents say their children's symptoms did not show up until after their children received the vaccines, required by many states for admission to school. If they prevail in the courts, the families are entitled to compensation from a multibillion-dollar trust fund.

The first of what eventually could be nine test cases from those claims is the subject of the hearing opening Monday in the U.S. Court of Federal Claims. Three special masters appointed by the court will preside over the hearing, expected to last through June 29.

The court is being asked to decide whether there is a link between autism and childhood vaccines. If it finds one exists, the families could be eligible for compensation under the Vaccine Injury Compensation Fund, a program established by Congress to ensure an adequate supply of vaccines by safeguarding manufacturers from lawsuits. Under the program, people injured by vaccines receive compensation through a special trust fund.

Autism is characterized by impaired social interaction. Those affected often have trouble communicating, and they exhibit unusual or severely limited activities and interests. Classic symptoms of mercury poisoning include anxiety, fatigue and abnormal irritation, as well as cognitive and motor dysfunction.

Monday's case addresses the theory that the cause of autism is the measles, mumps and rubella vaccine in combination with other vaccines containing thimerosal. The preservative, about 50 percent mercury by weight, is no longer found in routine childhood vaccines but is used in some flu shots.

In July 1999, the U.S. government asked vaccine manufacturers to eliminate or reduce, as expeditiously as possible, the mercury content of their vaccines to avoid any possibility of infants who receive vaccines being exposed to more mercury than is recommended by federal guidelines.