When my daughter Isabel's autism was diagnosed in 1994, when she was 2 1/2 , I knew little about the condition. Autism was a strange word to most people. "You mean like Dustin Hoffman in 'Rain Man'?" people would ask. "You mean she's artistic?"

Back then, autism was considered a rare disorder, occurring in only about three in every 10,000 live births.

Little more than a decade later, autism has become a "major public health concern," according to Marshalyn Yeargin-Allsopp, chief of the developmental disabilities branch of the Centers for Disease Control and Prevention. The results of a CDC survey released this month suggest that about one in every 150 children in the United States has some form of autism.

Those numbers don't reflect an epidemic, as some reports have suggested, or even mean that the incidence of autism has necessarily increased. Instead, we are defining autism and measuring its prevalence differently than we did in the past. Isabel's story illustrates that evolution.

The years since Isabel's diagnosis show a rapid change in our awareness of the disorder. In 1993, the state of Maryland (where we live) told the U.S. Department of Education that the state's public schools had provided special education services during the 1992-93 academic year to just 28 children between the ages of 6 and 21 with autism.

By the time Isabel was diagnosed, Maryland's public schools claimed to have served 300 people in the same age group with autism in 1993-94 (still a small number, but an enormous increase over the previous year).

And by 2003 there were more than 4,084 children ages 3 to 22 who had been given the official coding for autism in the Maryland public school systems, a rate of 1 in 183 children.

Isabel was no longer alone.

Shifting Definitions Like most of the illnesses psychiatrists treat, you cannot see autism under a microscope or discover it through a lab test. The only evidence we have is the individual's behavior. And until 1980, when the criteria we're familiar with today were first standardized in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM-III), there was little agreement about what was and was not autism. Even now, there is an element of subjectivity in each diagnosis.

What's more, many psychiatric diagnoses come and go -- and hence rates of diagnoses go up and down and change rapidly. Several million Americans have been diagnosed with depression, for example. But many of them would not have received that diagnosis just 50 years ago. The increase does not mean that depression is more common today than in the past, but only that our way of defining depression has changed. Half a century ago, the term was used to describe only debilitating depressions that required long-term hospitalization.

The shift in how we view autism is also part of a set of broader shifts taking place in society. The growth of child psychiatry after World War II, the rise of advocacy organizations, greater public sensitivity to children's educational problems and changes in public policies (the establishment, for example, of autism as a special-education code after the 1991 Individuals with Disabilities Education Act, or IDEA) have together changed the way autism is diagnosed and defined.

As a society, we have also become more aware of children's behavioral and learning differences at earlier and earlier ages and more comfortable with diagnosis, medication and psychiatric labels. Under the rubric of autism we now find a multitude of emotional and cognitive problems -- problems that used to be given other diagnostic labels or were even considered within the range of the normal.

And these days, researchers use reliable diagnostic tools to provide more accurate statistical counts of disease rates in a community, rather than basing their counts on reports of already known cases.

As a result, there are more people with a diagnosis of autism now than at any time in history.

And it's a better time than ever to be autistic.

More Awareness, ResearchWhat was our school supposed to do in the early '90s with Isabel, who was just one of a few hundred kids in the whole state labeled with autism? The key word here is "labeled," because there were certainly thousands of people in Maryland with autism at the time. So where were they? The answer is that many of the kids were in school; they just didn't have the label. That category had been introduced only in the 1991-92 academic year -- and then as an optional category.

A diagnostic label really does influence the way we view someone. If Isabel's condition had been diagnosed as schizophrenia, as might have happened in the 1950s and '60s, a psychologist might have recommended that she be sent to a mental institution or assigned her to a class or school for mentally disturbed children. If she had been diagnosed as mentally retarded, as so many autistic people were and still are, she would have been placed in classes for cognitively challenged kids.

Today, pediatricians, mental health-care practitioners, speech and occupational therapists and educators are providing therapy to children with autism at earlier ages than ever, and they are discovering how to use safe and effective medicines to ameliorate some of their symptoms.

Federal, state and local agencies have mobilized to manage the heavy public health burden of autism. Special-education programs are expanding; new money is pouring out of the National Institutes of Health into autism research; and donors are contributing millions of dollars to advocacy organizations, private schools and research foundations. Between 2003 and 2004, the number of grant applications to the National Alliance for Autism Research, which was then the leading private foundation for autism research, doubled.

The increased awareness, and the new, higher, more accurate rates, mean that a newly diagnosed child is no longer a mystery. And this is true no matter where you are -- in a suburb of Washington or in Seoul, Cape Town or New Delhi. Parents all over the world are beginning to break through the walls of stigma that had made them hide their children from public view. They are asking for the public assistance they deserve, and where it is absent they are demanding it.

Plenty of ProgressI trace the beginning of my knowledge about autism and Isabel's identity back to that beautiful spring day in 1994 when a short, slightly overweight, affable child psychiatrist at Johns Hopkins told my wife and me that "Isabel has enough features of autism to be called PDD-NOS, Pervasive Developmental Disorder Not Otherwise Specified. It means that she's not severely autistic." By that time we had already been worried about Isabel for more than six months. The discussion of her diagnosis was really just having someone we could trust tell us what we knew all along, even if we hadn't admitted it to ourselves.

Isabel was our first child, and so we didn't really have a standard for comparison. In her first two years, she had seemed like any other child, and at 12 months she had begun to make some of the sounds that seemed like the beginnings of words. We thought she was fine. But when I look at our home movies today, I see that she never tried to communicate with us; in none of the videos of Isabel between 18 and 24 months does she say a single word. At 25 months old, she made only fleeting eye contact.

She began flapping her hands and arms occasionally and didn't respond to her name consistently. She spoke little and couldn't even tell us if she was hungry. We were at a point where we demanded clarity from doctors. A valid diagnosis of what was wrong with Isabel was more satisfying than devastating. It gave us a framework for understanding Isabel and a road map for the future.

Today, my teenage daughter is mainstreamed into a high school classroom for part of the day. Numerous tests have shown that she has above-average intelligence. She plays cello in the school orchestra.

Just last week, I showed her two magazine articles about my new book on autism, in which I talk about how far she has come -- farther than my wife and I, or our doctors, ever expected. "What do these articles say?" I asked. "They say I'm doing great," Isabel replied. Indeed, she is. ·

Roy Richard Grinker, a professor of anthropology at George Washington University, is the author of "Unstrange Minds: Remapping the World of Autism" (Basic), from which this article is excerpted. Comments: health@washpost.com.