LOS ANGELES—The number of new cases of autism in California has fallen for the first time in more than 10 years in what may be a bellwether for autism rates nationwide, according to new data compiled by the state Department of Developmental Services.

The total number of autistic children receiving special education services from the state continues to grow - bringing the current total to 28,046 - but the rate of increase peaked in 2002 and has dropped slightly since then.

California has the best reporting system for autism in the United States because it guarantees special education services for autistic children. Because of its tracking system, the state is generally considered a predictor of what is to come for the rest of the country.

Experts said, however, that they don’t know what’s causing the numbers to fall off.

“Perhaps whatever caused (the number of cases) to go up ... is no longer present,” said Dr. Robert Hendren, executive director of the University of California, Davis MIND Institute, which researches neurodevelopmental disorders. “It’s all speculation. I wish we had good studies.”

Parent activist Rick Rollens of Sacramento, who played a key role in the creation of the MIND Institute, said that the trend roughly corresponds to the removal of mercury preservatives from pediatric vaccines. Many activist groups say that the use of mercury in the vaccines caused the sudden increase in autism cases.

Federal scientists, however, have continually said that the evidence does not support such a link.

The state data do not include children under the age of 3. About 90 percent of all autistic children are entered into the system before the age of 6, the department said.

According to the state data, 2002 was a record year for new autism diagnoses, with 3,259 cases. In 2003, the number of new cases slipped to 3,125. In 2004, the number was 3,074.

For the first half of 2005, there were 1,470 new cases, compared to 1,518 in the same period in 2004.