Experts aren't surprised by the growing numbers, and some say it simply reflects that doctors, teachers and parents are increasingly likely to say a child with learning and behavior problems is autistic. Some CDC experts say screening anddiagnosis are clearlymajordrivers, but that they can't rule out some actual increase aswell.

"We cannot say what portion is frombetter diagnosis and improved understanding versus if there's a real change," says Coleen Boyle, the CDC official overseeing researchinto children's developmental disabilities.

For decades, "autism" meant kids with severe language, intellectual and social impairments and unusual, repetitious behaviors. But the definitionhasgraduallyexpandedandnow includesmilder, related conditions.

One sign of that: In the latest study, almosthalfofautistickidshad average or above average IQs. That's upfroma thirdadecade ago andcan be taken as an indication that the autism label is more commonly given to higher-functioning children, CDC officials acknowledge.

Aside fromthat, muchinthe latest CDC report echoes earlier findings. Autism and related disorders continue to be diagnosed farmore often inboys thangirls, andinwhites than blacks or Hispanics. The racial and ethnic differences probably reflects white communities' greater focus on looking for autism and white parents' access to doctors, because there's no biological reason to believewhites get autismmore than other people, CDC officials said at a press briefing Thursday.

One change CDC officials had hoped to see, but didn't, was a drop in the age of diagnosis. Experts say a diagnosis can now bemade at age 2 or even earlier. But the new report says themajority of children continue to be diagnosed after they turn 4.

"We know the earlier a child is identified and connectedwith services, the better," Boyle says.

TheAmericanAcademy ofPediatrics issued a statement on Thursday, saying the nation needs to step up screening for the condition and research into autism's causes.

"It's critical thatwe as a society do not become numb to these numbers," saysDr SusanHyman, head of the group's autismsubcommittee.