The new research, by epidemiologists at the U.S. Centers for Disease Control and Prevention, found an incidence of 3.4 cases per 1,000 among children aged 3 to 10 in the Atlanta area.

"This overall rate is 10 times higher than rates from three other U.S. studies that used similar, specific criteria to identify children with autism and pervasive developmental disorders in the 1980s and early 1990s," the report says.

Pervasive developmental disorders are milder versions of autism, in which children have difficulty communicating and interacting socially. Dr. Marshalyn Yeargin-Allsopp, the CDC epidemiologist who is lead author of the new report, says the study, and others under way, are being done "because clinical colleagues began calling us to say they were seeing a lot of children with autism and developmental disorders."

The incidence found in the Atlanta study is similar to one reported in a recent New Jersey study and to those of several European studies, the report says. And California investigators released a report in October showing a sharp increase in rates of autism and related disorders in that state.

Autism has become a high-profile condition in recent years. Pressured by parents and advocacy groups, Congress provided funds to conduct studies about the condition in 2001 and 2002.

A number of studies designed to find a cause are going on, Yeargin-Allsopp says. They are "case-control" studies, in which a number of factors that might contribute to the condition are compared in children with and without autism. The results of one of those studies, also being done in Atlanta, could be published as early as this year, Yeargin-Allsopp adds.

Yeargin-Allsopp's new research, which appears in this week's issue of the Journal of the American Medical Association, lists several factors that might contribute to the increased incidence of autism and related disorders. They include greater pubic awareness of autism, changes in diagnostic criteria that have expanded the number of children listed as having a problem, and increased media coverage of affected children.

Yeargin-Allsopp's study does not mention one controversial hypothesis that has gained widespread publicity -- that the increase in cases is caused by the mercury-based preservative used in the vaccine for measles, mumps and rubella.

"This study was not designed to look at the causes of autism," Yeargin-Allsopp says. "There will be eight areas of the country where we look at prevalence and five areas where we examine the causes of autism."

A study released in October by researchers at the University of California, Davis reported that cases of autism in that state had tripled between 1987 and 1998. The authors of that study said the increase could not be explained by genetics, birth injuries, or changes in diagnostic criteria. It also found no evidence linking the vaccine to the condition.

The Atlanta study found that 40 percent of the children with autism and related problems were not diagnosed until they began school. Early diagnosis is important, particularly for pervasive developmental disorder, Yeargin-Allsopp says, because intensive therapy can enable those children to lead normal or near-normal lives.

Vanessa Collier, a spokeswoman for the Autism Society of America, says, "There is an alarming geographic growth in the incidence of autism in the United States and around the world. At these rates of growth, the cost of autism to the U. S. economy will be more than $300 billion through the next decade, constituting a national health crisis."

The cause of autism is still unknown "despite more than 50 years of study," Collier adds. There is "a split in the middle" among parents on the vaccine hypothesis, she says, noting, "Studies have not supported a link, and more research is needed."