Earlier estimates placed the rate at 1 in 166 children. But Catherine Rice, lead author of the analysis released Thursday by the U.S. Centers for Disease Control and Prevention, says the previous estimates were based on smaller studies, some from other countries, using different study methods.

Advocates for people with autism say the results should help them push for more services. The findings may further stoke the impassioned debate over causes: Many blame thimerosal, a mercury-based preservative that was used in infant vaccines up until six years ago. Scientists concede that they don't know the cause but say the science doesn't support a link to vaccines.

For the new CDC report, researchers drew on information gathered in 2000 and 2002 by a multistate surveillance network on 8-year-olds who were identified as having an autism spectrum disorder (ASD). The category includes autism, PDD-NOS (pervasive developmental disorder not otherwise specified) and Asperger's disorder. All are marked by problems with language and communication and often are accompanied by repetitive or unusual behaviors. ASDs begin before age 3 but may not be diagnosed until later.

Researchers focused on 8-year-olds because most children with such disabilities will have been identified by that age, Rice says.

The 2000 study, which examined children who were born in 1992, involved sites in six states and 1,252 children with ASDs. The prevalence averaged 6.7 children out of every 1,000.

The 2002 study, which focused on children who were born in 1994, involved 2,685 with ASDs at sites in 14 states and found that autism prevalence ranged from a low of 3.3 in 1,000 children in Alabama to 10.6 in 1,000 in New Jersey. The overall average was 6.6 in 1,000 children, or about 1 in 150.

Estimates differ because ASDs vary and some states have better diagnosis and tracking, Rice says.

Peter Bell, president of Cure Autism Now, says the report is "confirmation of our worst fears, and that is that autism is on the rise. Every two or three years, we're given an estimate that is higher than the previous one."

CDC Director Julie Gerberding said in a statement that it's still unknown whether there's an actual increase in autism or just better studies. "We do know, however, that these disorders are affecting too many children," Gerberding said.

States facing rising autism rates have struggled to provide services. Rice says the new information can be used by communities to help plan for the future needs of these children. "We know the best outcomes come when kids are identified early," Rice says.

The revised estimates will "increase awareness and hopefully get legislation to follow that will give services to those with autism," says Marguerite Colston of the Autism Society of America.