California's alarming increase in childhood autism is an authentic phenomenon that cannot be explained by flawed diagnosis, according to a new study that researchers say adds greater urgency to finding causes for the explosion in cases.

The study, released to state legislators Thursday, found that a 273 percent jump in cases could not be explained by loosening of criteria describing the developmental disorder, misdiagnosis or an influx of people from other states. The study was commissioned following the steep rise in cases at 21 state-run treatment centers from 1987 to 1998.

"I am somewhat surprised by the results," said UC Davis pediatric epidemiologist Robert Byrd, the lead investigator. "It's sobering that we have to face these numbers."

And the numbers continue to spiral upward, with no clear cause, Byrd said. Autism is now the fastest-growing segment of the state's developmental disability system.

State figures show that more than 6,500 new autism cases were seen at the treatment centers between 1999 and 2001 -- about the same number of new cases seen in the entire period from 1970 to 1995. Two-thirds of them were children 13 or younger.

A number of Bay Area school districts now have entire classes filled with youths who have forms of autism. San Francisco has its own autism curriculum specialist. Parents around the state are scrambling to find the best therapies,

often finding long waiting lists at clinics.

"Clearly this says that California is in the midst of an autism epidemic," said Rick Rollens, a parent advocate who co-founded a research group at UC Davis called the Medical Investigation of Neurodevelopmental Disorders Institute, which was asked by the state Legislature to study the numbers. "Many in mainstream medicine are afraid to use the term epidemic."

The institute, though co-founded by Rollens, is an independent research center.

MORE RESEARCH URGED

State Sen. John Burton, D-San Francisco, who pushed for legislation authorizing the study, said it adds urgency to the search for a cause. "There is more awareness, more people acknowledging it," he said. "We've come a long way toward recognition, but this study shows there is an awful lot of work and research that has to be done."

Until the 1990s, autism wasn't widely recognized by the public. The 1988 Dustin Hoffman movie "Rain Man" gave most people their first glimpse of the disorder, which is characterized by difficulty in communicating and forming social relationships.

Autistic children can have trouble making eye contact, picking up emotional cues or interpreting facial expressions. Some develop obsessive interests or repetitive self-stimulating behaviors like rocking or flapping their hands.

Now, researchers describe a range of autistic behaviors, from the full- blown disorder to milder forms like Asperger's Syndrome. Children with Asperger's, who can become trivia experts capable of reciting the phone book but not of making friends, weren't included in the 1987 to 1998 data.

Byrd's research team, which conducted the study for the M.I.N.D. Institute at UC Davis, surveyed families of 684 children who received services from regional centers run by the state Department of Developmental Services. Researchers looked at children in two age groups, from 7 to 9 and from 17 to 19. They surveyed parents about prenatal and early childhood events, and reviewed center data.

The increasing numbers have led to many theories. Some therapists and researchers have believed that the changing definition of autism in psychiatric diagnostic manuals led to more kids being labeled autistic.

Others have postulated that newcomers to California accounted for the increase, or that parents of mentally retarded kids were seeking an autism diagnosis because it promised better and earlier interventions.

The study did not back up those theories. The centers' accuracy in diagnosis was about 90 percent for both groups of kids, the study found. Although the criteria for autism has changed, said Byrd, the greatest alterations happened before the period covered in the study.

The report also said that more than 90 percent of the kids in the survey were born in California. And there was no notable change in the percentage of mentally retarded children misclassified as autistic through the years covered by the study.

VACCINATION THEORY

Byrd said more study needed to be done in cases where kids were classified as mentally retarded without evidence of autism because most of their families did not respond to the survey.

Now that researchers know the numbers show a true trend, Byrd said, attention needs to be focused on the causes of autism. Multiple studies are under way, both nationally -- where the trend is also evident -- and in California.

One area of study is the theory that childhood vaccinations contribute to autism. Rollens, who lives near Sacramento, said his son Russell began life in 1991 as a normal, robust infant, then slipped into autism after his first vaccination at seven months.

The condition worsened shortly after that when Russell got his first measles, mumps and rubella shot. Rollens is convinced his son has vaccine- induced autism. Other parents have described similar scenarios in which their children had vaccinations, then regressed after developing normally.

The UC Davis study showed that a third of parents of more recently diagnosed kids thought symptoms were caused by vaccinations. Only about 18 percent of the parents whose kids entered the system earlier believed vaccinations were behind the disorder.

When asked what caused autism in their kids, parents in both groups most commonly answered that they didn't know.

Vaccine-related autism may exist, Byrd said, but a sudden "step increase" would be expected if vaccinations were the "driving force" behind autism numbers. But that doesn't mean that vaccines, or other environmental factors or genetics, aren't causing autism. The problem is that experts can't definitively say if they are -- or in what combinations.

"I think this will once again ignite interest to fight the epidemic," Rollens said. "This is a severe public health emergency. We need to find out what is causing it and to find prevention."