About 1 in 88 children in the United States has autism and the prevalence of the condition has risen nearly 80 percent over the past decade, federal health officials reported Thursday.

The survey, sponsored by the Centers for Disease Control and Prevention, is the latest evidence of a steady upward trend in a disorder whose cause remains unknown despite much research in recent years.

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Children with the most extreme form of autism are socially withdrawn, speak little, dislike affection and eye contact, and engage in repetitive actions.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.

Most parents of autistic children — and their advocacy organizations — strongly dispute the idea that a looser definition of autism and more assiduous search for mild cases explains the trend.

“Autism is now officially becoming an epidemic in the United States. We are dealing with a national emergency that needs a national plan,” said Mark Roithmayr, president of Autism Speaks.

Invited by Frieden to speak at the briefing, Roithmayr respectfully disagreed with him.

“Only part of the increase is better diagnosis. Something is going on here that we don’t know,” Roithmayr said. He added that “we desperately need deeper and broader funding” for treatment of the disorder and research on its causes.

In recent years, some groups have blamed autism’s rise on a preservative, thimerosal, that is no longer used in childhood vaccines and on the practice of giving infants multiple immunizations at a single visit. Research has ruled out both as a cause.

CDC has several studies underway assessing genetic, reproductive, environmental and behavioral variables associated with autism. One of them, the Study to Explore Early Development (SEED), is following 2,700 children and their families in six states, including Maryland.

As soon as the new prevalence estimate was announced, advocacy groups jumped on the news to promote their theories and agendas.

“These stunning new figures are a call to action among our elected leaders to minimize our children’s exposures to mercury and other toxic chemicals,” said Ken Cook of Environmental Working Group. “Upending the federal government’s approach to regulating toxic chemicals and putting tough emissions standards in place at power plants are two good places to start.”

Research suggests that some families have a genetic predisposition for autism and that parental age may play a role. There’s also evidence the condition may begin before birth and be the end result of a disordered process of brain development. A recent study in mice showed that an autism-like disorder could be reversed by bone-marrow transplantation that restored a certain type of cell, called microglia, to the brain.

In the CDC study, surveyors looked for 8-year-olds who were getting special education services or were being treated at specialty hospitals or medical units for one of three “autism-spectrum” disorders. The children’s records were then reviewed by clinical experts to see that they actually fit the diagnosis. The children themselves, however, were not examined.

Among the children with a diagnosis in their medical records, 44 percent had autistic disorder (which is at the severe end of the spectrum); 47 percent had pervasive developmental disorder; and 9 percent had Asperger syndrome. The prevalence of all three subtypes has been rising in about the same proportion, according to the survey.

If the rising prevalence represents an actual increase of the disorders — and is not the consequence of finding previously undiagnosed cases — that suggests there may be environmental exposure or demographic change (such as older parenthood) that is responsible, because a population’s genetic background wouldn’t change over a few decades.

The autism prevalence in Maryland is 12.9 per 1,000 children at age 8, slightly above the national average. The disorder is slightly more common in white children than in blacks, but twice as common in whites as in Hispanics.

The survey was done in six counties: Anne Arundel, Baltimore, Carroll, Cecil, Harford and Howard. It did not include Baltimore City, where the three hospitals with special programs for the metropolitan area are located.

Lee, who is affiliated with Johns Hopkins University’s Bloomberg School of Public Health, said that Baltimore City was surveyed the first year of the survey but was dropped because principals in the city schools didn’t have enough time to cooperate with researchers.