PHOENIX — Siding with parents over insurance companies, Gov. Janet Napolitano signed legislation Friday which eventually will require insurance companies to provide coverage for autism. But there will be limits.

 Napolitano’s decision caps months of intensive lobbying by parents of children who have been diagnosed with autism and varying related conditions to finally get some financial help from insurers.

 The move came despite lobbying from not only the insurance companies but business interests. Marc Osborn who represents the Arizona Chamber of Commerce and Industry, predicted the new mandate will lead to higher insurance costs for business — and even the possibility the additional charges will result in some firms deciding they can no longer afford to provide coverage.

 But Osborn found a hostile audience, even among otherwise business-friendly lawmakers who said they wouldn’t be enacting legislation if insurers worked with parents. Gretchen Jacobs, the mother of a 4-year-old girl with autism, rejected the contention that the law, which takes effect in mid 2009, amounts to a new mandate.

 She said insurers refused to provide coverage for children with autism or even its milder form, known as Asperger’s syndrome, on the basis that it was not treatable. She said more recent studies show that is not the case. “It’s less of a mandate, more of an update,’’ Jacobs said.

 Statistics from the Centers for Disease Control show that one out of every 150 children are affected with some form of autism which can manifest itself as an impairment in thinking, feeling, language and the ability to relate to others. The CDC said that it appears to be far more common in boys than girls.

 The state Department of Developmental Disabilities provides coverage for about 3,160 children with the most severe form of autism. Other forms are not covered.

 Jacobs, who used to lobby on behalf of insurance companies, said evidence now shows that children do respond to certain types of treatment. But it can be costly, which is why the parents want coverage.

 Osborn, in testifying against the bill before the Senate Health Committee, said that is shifting the cost from DDD to insurers — and, ultimately, to the companies that pick up the tab. That brought an angry response from Sen. Barbara Leff, R-Paradise Valley. “If you have insurance, your employer pays for insurance, your baby has a heart condition, you’re going to pay for that surgery,’’ she said. Leff said autism “is still an illness.’’

 Lawmakers did agree to some limits. First, the mandate applies only to employer-purchased insurance for companies with at least 50 workers under the premise that these firms, with a larger base, are better able to absorb the cost.

 Second, benefits would be limited to $50,000 a year for children through age 8; those 9 through 16 would have benefits capped at $25,000 a year.

 Jacobs said the higher figure for younger children makes sense as there is evidence that early and intense intervention can make a difference.

 Less clear is what causes autism. There has been some claims that mercury-based preservatives used in some children’s vaccines has an effect. Jacobs said even though most vaccines no longer have that chemical, other environmental factors are suspected.

 But she said the increased number of children diagnosed with autism could also be due to a “greater awareness’’ by doctors and others of the condition — and the willingness to make that diagnosis.

 In her own case, Jacobs said her daughter was developing ahead of schedule until she turned 14 months when she got sick and “pretty much deteriorated over a two, three week period.’’ Jacobs said her daughter stopped talking, lost interest in family and toys and even stopped chewing her food “and we had to start pureeing all of her food.’’ “I didn’t realize you could ‘get’ autism,’’ Jacobs said. “I always thought you were born with it.’’

 Sen. Tom O’Halleran, R-Sedona, who voted for the measure in the Health Committee, said the issue of autism goes beyond the families and the insurance company. “It is the problems of America and the problems we are having with our health care system,’’ he said.