By LAURIE TARKAN

No one has found a cure for autism, the neurological disorder that leads to lifelong impairments in a child's ability to speak, respond to others, share affection and learn. But there is a growing consensus that intensive early intervention is both effective and essential — the sooner after diagnosis, the better.

Early intervention, which involves many hours of therapy with one or more specialists, does not help every autistic child to the same degree. It is best started no later than age 2 or 3, and for reasons that are unclear, it does not help some children at all. But for those who are helped, their parents say, the changes are miraculous.

Yet the success of early intervention is posing a painful predicament for schools and families — a predicament made more immediate by a rising tide of diagnoses of autism. Last week, researchers reported that the number of austistic children in California had risen more than sixfold since 1987, and other states and the federal government have also noted sharp increases.

By federal law, public schools must provide appropriate education for children with disabilities, starting at age 3. But the treatment is so expensive — averaging $33,000 a year, according to research published in the journal Behavioral Intervention — that many families cannot persuade their school districts to pay for it.

Brian and Juliana Jaynes of Newport News, Va., can testify to that. As a baby, their son, Stefan, developed normally, if not ahead of the curve. By age 2, his vocabulary was well over 100 words. He knew his address and his colors, and he spoke in short sentences. But soon after his second birthday, he started to regress, forgetting the words he once knew.

His parents suspected a neurological disorder. A specialist confirmed their suspicions, telling them Stefan was severely autistic and urging them to get intensive therapy for him.

Instead, school officials placed Stefan in a special-education preschool, where, the Jayneses say, he rapidly regressed. (The school district says the placement was appropriate.) After the neurologist told the frantic couple that their son might have to be institutionalized, they removed him from the preschool and began 40 hours a week of behavior therapy at home.

It cost them more than $100,000 over three years. Today, Stefan, 11, attends a school for autistic children and has vastly improved his language, social and self-help skills. He can say some simple sentences and communicate his needs; perhaps most important, he spends more and more time interacting with his family, and less time in his own world. The behavior therapy, his father said, "has brought about an awakening in this little boy's personality that is truly a miracle."

In recent years, four leading institutions — the American Academy of Pediatrics, the American Academy of Child and Adolescent Psychiatry, the Surgeon General and the National Academy of Sciences — have called for early intervention, including one-on-one therapy, for children with autism. A panel of experts convened by the academy last year recommended a minimum of 25 hours a week, 12 months a year.

But Dr. Catherine Lord, the panel's chairwoman and a psychology professor at the University of Michigan, estimates that fewer than 10 percent of children with autism are getting the recommended level of therapy. "Almost everywhere, schools will say kids are getting services," she said. "But what they're getting varies enormously."

Because the young nervous system has a great deal of plasticity, many experts believe that early intervention enriches neural growth.

Dr. David L. Holmes, president of the Eden Institute, an autism center in Princeton, said, "If you have a child with autism who's not wired correctly, and we allow that to continue without intervention, those neuropathways will become fixed, and it becomes far more difficult to undo that tangled mess."

Autistic children lose the ability to learn by observation, something other children do constantly. Behavioral therapy is aimed at teaching these children how to learn. Teaching an autistic child to wave goodbye, for instance, can take 40 hours of repetitive lessons.

There are several kinds of therapy. The most popular — the one Stefan Jaynes receives — is applied behavioral analysis, in which a therapist asks a child to perform small tasks and then offers feedback to reinforce correct responses.

Other programs use sensory integration therapy, based on the theory that autistic children have defects in processing the messages from their five senses; auditory integration therapy, which assumes that some are oversensitive or undersensitive to sound or have problems processing sounds; speech therapy; and group programs.

The federal education law leaves decisions about therapy to professionals and parents. But administrators say parents often demand far more therapy than the experts recommend. "Is the school system going to override teachers, and substitute the teacher's decision with the parent's decision?" asked Bruce Hunter, associate executive director for public policy at the American Association of School Administrators in Arlington, Va.

The biggest obstacle is budgetary. "When you're looking at limited resources in a school district, sometimes the available resources drive what services schools will propose to offer," said David Egnor, policy director at the Council for Exceptional Children. "It's simply pragmatic."

Mr. Hunter added: "The problem all along in special ed is that you have a chronic shortage of money that is exacerbated by downturns in the economy, which is when it really gets bad. You get the joy of taking the money from one group of children and spending it on another group."

Under law, the federal government may reimburse states up to 40 percent of the extra cost of educating a child with a disability. But this year, Congress is paying just 17 percent, or $7.5 billion. President Bush has proposed adding $1 billion next year.

"The federal and state governments ought to pay attention to these children who have disabilities and need to be educated and need special treatment, and that costs money," said Representative Dan Burton, Republican of Indiana, who has an autistic grandson.

But the chairman of the House Committee on Education and the Workforce, John A. Boehner, Republican of Ohio, opposes full financing of the act until major changes are made. He and others have called for reforms in identifying students with disabilities — minority students are classified far out of proportion to their numbers — and in the daunting paperwork for the schools.

Many experts believe society would pay less in the long run if children received appropriate early intervention. An article in Behavioral Intervention in 1998 found that if 100 children were given early intensive intervention and 40 of them had only partial improvement, the public would save $9.5 million over their school years, ages 3 to 22.

Most insurance companies do not pay for therapy for developmental disorders like autism, though a few companies offer reimbursement as part of their health benefits.

Another obstacle to treatment is a lack of specialists. Public schools have a shortage of more than 12,000 special education teachers, and the number is expected to grow as many teachers retire or leave the field.

Advocates say the supply of teachers trained to deal with autism is even shorter, so schools are forced to rely on expensive outside specialists.

Even parents who decide to pay for treatment have trouble finding private specialists. Autism schools and private behavioral therapists typically have waiting lists of more than a year. This forces parents to set up their own in-home school and hire teams of people to provide the 20 to 40 hours a week of therapy. Many parents train themselves in the behavioral therapies, and then train college students, whom they can hire for considerably less money than specialists.

Yet another obstacle to early intervention is delayed diagnosis. Autism is most commonly diagnosed at 20 to 36 months, but experts say the signs often surface earlier. Many families experience delays because pediatricians often dismiss their concerns.

The growing awareness of autism may ease that problem. (Autism is now diagnosed in 1 out of 600 children, by most estimates.) But without appropriate therapy, early diagnosis does little but create frustration for parents, as Stefan's mother, Juliana Jaynes, recalled recently. "I had the doctor telling me that every moment counts," she said. "There's that horrible feeling of time slipping away and nothing being done."