The number of autistic people in California has nearly doubled in the past four years, alarming parental advocates and renewing pressure to find a cause.

On Tuesday the state Department of Developmental Services released its first comprehensive report on autism since a widely published study in 1999. The report concludes that the autism caseload in the state continues to accelerate with no sign of slowing.

The number of autistic people receiving services through the state's regional centers jumped from 10,360 in December 1998 to 20,377 in December 2002.

That's a 97 percent leap in four years, and a stunning 634 percent rise since 1987.

Even some of the people closest to the numbers say they are taken aback at the trend.

"I find it very frightening that it could go up at this magnitude and we still don't know why," said Bob Baldo, executive director of the Association of Regional Center Agencies. "From the human- tragedy standpoint and the cost standpoint, we need to find out what is going on."

A similar trend has been observed in Washington state. Autism cases have increased in recent years by more than 130 percent, according to officials of the Autism Center at the University of Washington's Center on Human Development and Disability.

Ron Huff, the senior psychologist who authored the California study, noted that when the 1999 numbers came out, many people believed that changes in the definition of autism and improved diagnosis were the cause of the rapid increase. If that is true, the numbers should plateau at some point.

"We had been monitoring the numbers thinking all along that they would decrease from the time we did the 1999 report," Huff said. "But after about three years, we realized that the numbers were actually going up and seemed to be going up at a higher rate."

Once a rare disorder, autism is now more prevalent than childhood cancer, diabetes and Down syndrome in California.

Autism is a severe developmental disorder that undermines a child's ability to connect with the world. Autistic children often have difficulty making eye contact and carrying on a conversation. Many engage in ritualistic behavior such as hand-flapping and following routines.

There is no known cause and no cure.

Most experts believe there is a genetic component to autism. But some researchers and parents suspect that an unknown environmental factor such as a toxic substance, pesticides or a bad reaction to a vaccine may trigger autism in genetically vulnerable children.

The study found that in 1970, there was one case of autism for every 2,500 births in California. By 1997, that had increased to one autistic child per 323 births.

"This is not just a California phenomenon," Huff said. "This is a global phenomenon.

"The story here is really about the families. You have to appreciate the impact that this disorder has on families."

Nearly 70 percent of people with autism are now less than 15 years old, compared with 35 percent in 1987.

And 97 percent of such children live at home with their parents, the study found.

Information from Seattle Times staff writer Warren King is included in this report.

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