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She believes there's a true increase in the prevalence, not just better detection.

"If you had made a change in diagnosis to be more sensitive, then I'd say we're getting better," she said. "The rates have rapidly increased in the last 20 years. The CDC is collecting existing data on kids who have been identified. If you went out and tested every child, the numbers would be even higher."

Only part of the increase should be attributed to better diagnosis, agreed Mark Roithmayr, president of Autism Speaks, a leading autism science and advocacy organization. "There is a great unknown. Something is going on."

Nationally, autism spectrum disorders are about five times more common among boys than girls -- 1 in 54 boys, compared with 1 in 252 girls. Symptoms typically are apparent before age 3. They include impaired social interaction and communication, repetitive patterns of behavior, such as hand flapping or rocking, and obsessive interests.

Data collection in the CDC study focused on 8-year-olds because that's the age of identified peak prevalence.

The number of children identified with autism spectrum disorders varies widely across the country, ranging from 1 in 210 children in Alabama to 1 in 47 children in Utah. Wisconsin's autism prevalence falls in the middle.

"One thing the data tells us with certainty -- there are many children and families who need help," Frieden said. "We must continue to track autism spectrum disorders because this is the information communities need to guide improvements in services to help children."

Kids diagnosed earlier

The new study shows more children are being diagnosed earlier, by age 3, an increase from 12% for children born in 1994 to 18% for children born in 2000. The median age at diagnosis has improved from age 4.5 to age 4 in recent years, according to Boyle.

Earlier screening is necessary, at ages 18 months and 24 months, so children can receive services for autism sooner, Frieden said.

"Many children are not receiving services early enough, or consistent enough, to reach their full potential," he said.

"For parents, it's critical to act quickly if there's a concern about your child's development," Boyle said.

Tracking autism prevalence helps researchers identify potential risk factors, such as advanced parental age and premature birth, Boyle said. The CDC recently finished the first phase of the largest study of autism risk factors in the U.S., she said.

"To understand more, we need to keep accelerating our research into risk factors and causes of autism spectrum disorders," Boyle said.

In response to the new CDC autism numbers, Autism Speaks on Thursday called for development of a national "autism action plan" that includes increased funding for basic science to uncover the genetic underpinnings of autism; increased funding for environmental research to detect causes of autism; and accelerated funding and development of effective medicines and treatments.

Autism resources

Autism Speaks: www.autismspeaks.org . Autism Speaks offers a free 100-day kit, personalized to locations, that explains how to handle the first 100 days of a new diagnosis.

Centers for Disease Control and Prevention: www.cdc/ncbddd/autism/facts.html

U.S. Department of Health & Human Services: www.hhs.gov/autism/