Researchers say they are baffled by the skyrocketing number of autism cases among California children.
The rise has little to do with evolving diagnostic methods and population increases, according to a study by the University of California, Davis.
A previous report showed an increase of 273 percent in the number of reported autism cases in California between 1987 and 1998.
"It is astounding to see a threefold increase in autism with no explanation," said Dr. Robert Byrd, an epidemiologist who led the study. "There's a number of things that need to be answered. We need to rethink the possible causes of autism."
Autism affects a person's ability to communicate, form relationships with others and respond to their environment.
The study, funded by $1 million in state money, collected data from 684 autistic children throughout California. Researchers found that changes in diagnosing autism over the years and population growth did not account for the increasing number of cases.
The next obstacle is to pinpoint a cause for the outbreak, Byrd said. Researchers have discussed several possible causes, including childhood vaccinations and genetics.
"All these things are out there without anything saying what these numbers really mean," he said.
Some researchers said they believed the autism numbers are even higher.
Dr. Bernard Rimland, director of the Autism Research Institute in San Diego, said the UC Davis study dramatically undercounted autism cases.
"I certainly am pleased that they finally acknowledge that there is an increase," Rimland said. "However, the numbers they gave are nonsense. They're kind of minimizing it."
The report was delivered to state legislators and other officials Thursday, and researchers said they will press the state to quickly approve a study on possible causes.