Advocates for children with autism are celebrating the passage of an insurance mandate that will require some — but not most — Minnesota health plans to cover an intensive form of therapy that can cost $100,000 a year.

The mandate, which was approved by the Legislature over the weekend, is scheduled to take effect in January.

Most Minnesota health insurers have refused until now to pay for the therapy — known as early intensive behavioral intervention — because of concerns about its cost and effectiveness.

But advocates lobbied heavily to change that, with dozens of supporters in red “Autism Votes” T-shirts flooding hearings at the Capitol in recent weeks.

“It’s terrific,” said Eric Larsson, a Minneapolis psychologist who runs the Lovaas Institute Midwest, an autism treatment program. He called it especially good news for families who were in danger of losing coverage for their children’s therapy.

In practice, the mandate will apply only to a fraction of Minnesota employers — those with 50 or more employees in state-regulated health plans.

The mandate would cover 750,000 Minnesotans, or about 14 percent of the state population, according to state estimates. But it does not apply to the vast majority of large employers, which are self-insured and exempt from state insurance regulation.

Even so, Larsson says that the mandate will start “chipping away at the iceberg,” and that many other groups are likely to start covering it voluntarily.

The mandate is likely to drive up health insurance costs for the affected companies, said Jay McLaren, director of government relations for Medica Health Plans. He predicted that it will give employers another reason to switch to self-insurance to avoid state mandates.

Under the new rules, state-regulated health plans would have to pay for any autism treatment that’s considered medically necessary for children up to age 18. The most intensive programs provide one-on-one therapy for up to 40 hours a week, often in the child’s home.

Minnesota is the 33rd state to adopt an autism insurance mandate, according to the national advocacy group Autism Speaks.

The Legislature also voted to authorize the same coverage under the state’s Medical Assistance program for low-income or disabled children up to age 18, and to reduce parental fees for the program. That provision requires federal approval.

Gov. Mark Dayton is expected to sign the bill later this week. It is part of the omnibus health-finance package approved by the House and Senate late last week.