In 1998, British researchers suggested a link between autism and the measles-mumps-rubella vaccine. There's been a subsequent backlash against vaccines in England - and recent measles outbreaks.

The theory has gained increasing attention in the United States, along with another theory linking autism and thimerosal, a mercury-based preservative used in some vaccines.

The controversy even ended up in a debate over a provision attached to Homeland Security legislation enacted last fall. The provision, aimed at protecting drug makers from lawsuits over vaccine-related injuries, prompted vocal protests in Washington in January by parents who see a connection between vaccines and autism.

The Institute of Medicine reviewed the issue and in 2001 said there was no proof that autism is caused by the MMR vaccine or thimerosal.

But vaccine foes note the IOM report said a link between thimerosal and an increased risk of neurodevelopmental disorders is "medically plausible."

Dr. Neal Halsey, an influential vaccine proponent from Johns Hopkins University, agrees that thimerosal could theoretically be linked with subtle developmental problems including delayed speech, "but the available data show no evidence of an association with autism."

Dr. Patty Manning-Courtney, a developmental pediatrician specializing in autism with the Kelly O'Leary Center for Autism Spectrum Disorders at Cincinnati Children's Hospital Medical Center, doubts the vaccine connection.

Enquirer reporter Peggy O'Farrell contributed to this report.