The rising rate of autism could be the result of finding children missed in earlier surveys or an actual increase in the condition — or a combination of the two. The trend has been observed in Canada and Western Europe as well as the United States.

Children with the most extreme form of autism are socially withdrawn, speak little, dislike affection and eye contact, and engage in repetitive actions. Once thought to be very rare, milder forms are now recognized. One of them, Asperger syndrome, describes bhavior that in the past might have been seen as peculiar and abnormal but not evidence of illness.

The CDC study surveyed 14 states — including Maryland — for the prevalence of autism spectrum disorders among 8-year-olds in 2008. The prevalence that year of 11.3 cases per 1,000 children was 23 percent higher than in 2006. It was 78 percent higher than in 2002, when the survey began. Autistic children received their diagnosis at age 4 on average — six months earlier than in 2006, but not early enough for optimal therapy, according to many experts.

The survey found large unexplained differences between sexes, among ethnic groups and in states.

For example, autism is five times as common in boys as girls (a lopsided ratio found in many other studies). The fraction of autistic children with average or above-average intelligence has risen more than the fraction with “intellectual disability.” Autism prevalence in Hispanic children is two-thirds that of white children, but it is rising faster in them and in black children than in white ones. The prevalence in Utah’s children is four times that in Alabama’s.

Such variation suggests that better identification of autism cases contributes to the higher number, but whether it explains the trend completely is a matter of huge debate.

“This is the billion-dollar question, isn’t it?” said Li-Ching Lee, an epidemiologist who headed the survey in Maryland.

In a telephone briefing with reporters, Thomas R. Frieden, CDC’s director, said the increase could be “the result of better detection. It is a possibility.”

What’s not a matter of debate, he said, is that autism is common and that doctors and teachers need to find children with it earlier, when treatment is most effective.

“There are many children and families who need help. There are many children who are not receiving services early enough or consistently enough,” Frieden said.

Autism often has devastating effects on families, and treatment requires time, skill and extreme patience. Medical expenses for children with autism are six times as high as those for children without the disorder. Behavioral therapy, often delivered one-on-one, can cost as much as $60,000 per year. The advocacy group Autism Speaks estimates autism spectrum disorders costs the United States $137 billion a year.