An explosion in autism cases in California over the past 15 years is not the result of changes in diagnostic criteria or an improvement in diagnosis, but represents a real epidemic that is sweeping the state, according to a major new study sent to the state Legislature on Thursday.

The number of people with autism being served by the California Department of Developmental Services surged by 273% from 1987 to 1998 -- and is still growing by about nine cases per day. That increase cannot be explained away by better data, immigration to the state or any other simple rationale, the report said.

Experts believe the epidemic is affecting the rest of the country -- and most industrialized nations -- as well, but California is a bellwether because it is the only state for which good data are presently available.

"Autism is on the rise in the state and we still do not know why," said Dr. Robert S. Byrd of UC Davis' MIND Institute, the primary author of the report. "The results of this study are, without a doubt, sobering. They increase the urgency of trying to find an answer about what causes autism."

Experts said the report should quell the controversy over whether the epidemic is real.

"This study will prove to be a landmark in that it clearly dispels many myths and misconceptions regarding the reality of the widely discussed epidemic of autism," said Bernard Rimland of the Autism Research Institute in San Diego. "The epidemic is very real. There has been an enormous increase in the prevalence of autism, and the increase cannot be explained by artifacts."

The growth of autism cases in California has been startling: from 4,911 cases in 1993 to 18,460 cases as of July. Of that latter number, 13,935 are children younger than 18.

The surge in cases forced the department to seek additional funding of $17.2 million this year to treat all of them, according to Cliff Allenby, director of the Department of Developmental Services. "This is really straining our resources," he said.

California has been particularly affected by the epidemic because it is required by state law to provide services to any resident diagnosed with autism or other mental disorders, Allenby said.

Services in other states "are limited by their budgets," he added. The increase may well be happening there also, "but they don't see it."

Autism is a severe developmental disorder in which children seem isolated from the world around them. There are a broad spectrum of symptoms, marked by poor language skills and an inability to handle social relations. No cure exists, but many problems can be alleviated with intensive behavioral therapy.

Byrd's three-year study was commissioned by the Legislature after a 1999 report of the 273% increase in autism cases over a decade. During the same period, in contrast, the department's enrollment of children with other disorders, such as epilepsy or cerebral palsy, increased 30% to 40%, consistent with the growth in the state's population.

His team enrolled 684 children who receive services from the department, dividing them into two age groups 7 to 9 years of age and 17 to 19, representing the periods before the rise in new cases and after it. The team found that 375 children had autism and 309 had a diagnosis of mental retardation.

Comparing the two groups, the team concluded that there had been no loosening in the criteria used to diagnose autism. Furthermore, more than 90% of the children in both groups were born in the state, eliminating the possibility that the increase was the result of families coming to California to seek better services.

"This is an important first step because it removes the suspicion that things are changing in terms of diagnoses," Allenby said. "Now it is incumbent on investigators and us to try to find out why it is happening, because we don't know yet."

Genetics clearly plays a role. Researchers have so far identified at least half a dozen genes that predispose people to the development of autism. But virtually everyone agrees that genes are not the sole answer. "Genes don't cause epidemics," Byrd said.

Many parents believe that vaccines are at the root of the problem. Some think the measles-mumps-rubella (MMR) vaccine is the primary villain, some maintain that a mercury-based preservative in the vaccines is the problem, and others argue that the 20 or more vaccinations infants receive by the age of 2 simply overwhelm their immune systems.

But "there is absolutely no evidence for that," according to Dr. Robert Edwards of UC San Francisco.