About 300,000 U.S. children have been diagnosed with autism, according to the largest national study so far of the prevalence of this complex behavioral disorder.
That means about 5.5 out of every 1,000 school-age children have been diagnosed with autism.
Past estimates have ranged from 1 to 9 out of every 1,000 children, based on smaller studies in individual states or cities.
The government-run study released Thursday reports findings from national surveys of tens of thousands of families.
The study by the U.S. Centers for Disease Control and Prevention also found boys are nearly four times more likely than girls to be identified with the condition. And it found Hispanics had lower autism rates, though it's possible that may be related to health-care access problems.
"There's somewhat of a dearth of information how autism impacts children and their families," said lead author Laura Schieve, an epidemiologist with the CDC.
Because of its national scope, the CDC report "is probably one of the best" studies of how often autism is diagnosed, said Dr. Eric Hollander, an autism expert at New York's Mount Sinai School of Medicine.
The study does not attempt to answer whether autism is increasing — a controversial topic, driven in part by a debate over whether autism is linked to a vaccine preservative. The new research is being published this week in the CDC publication, Morbidity and Mortality Weekly Report.
This is the first in a series of autism studies being issued by the Atlanta-based CDC. Future articles will look at autism diagnosis variations across different communities, and at how long it takes for a child to be diagnosed after onset of symptoms.
Autism is a complex disorder usually not diagnosed in children until after age 3. It is characterized by a range of behaviors, including insistence on sameness, difficulty in expressing needs and inability to socialize.
Schieve's study pulls together results from two surveys done in 2003 and 2004. In both, parents were asked the same question: Has a doctor or health-care provider ever told you your child has autism?
In the first survey, conducted through personal interviews, 102 of the 18,885 children in the sample were identified as autistic. When the numbers were statistically adjusted to account for families that didn't respond and to better represent the U.S. population, the resulting prevalence rate was 5.7 per 1,000.
In the second survey, done through random-digit-dialed telephone questioning, 465 of the 79,590 children in the sample were identified as autistic. The adjusted rate was 5.5 per 1,000.
Researchers believe some parents may have answered 'yes' for two similar but less severe diagnoses, Asperger disorder and pervasive developmental disorder (which is sometimes called "atypical autism").
For that reason, the study's prevalence rates may reflect other autism spectrum disorders and not just autism alone, CDC officials said.