SAN FRANCISCO - The number of autistic children in California has nearly doubled in the last four years, a new state report shows.

The report found that 20,377 children ages 3 and older were receiving state services for "classic" autism, a developmental disorder characterized by difficulties communicating and repetitive actions, as of December 2002. That was a 97 percent increase over the 1998 number.

An earlier study had found the state's autism rate increased 237 percent from 1987 to 1998.

Ron Huff, a psychologist who conducted both studies for the California Department of Developmental Services, said the latest numbers reflect an actual increase, not just changes in the diagnosis.

Despite greater awareness and more research in recent years, the report said scientists still do not know what causes the developmental disorder.

State researchers are investigating potential genetic and environmental toxins for links to autism. They are also exploring the debated theory that childhood vaccines may be at the root of the problem.

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