utism is about 10 times as prevalent today as it was in the 1980's, according to the country's largest study ever on the problem. Some of the increase is the result of widened definitions of the disorder, researchers say, but the explanation for the rest of the increase is unknown.

The study, conducted in metropolitan Atlanta in 1996, found that 3.4 in every 1,000 children ages 3 to 10 had mild to severe autism that year. In the late 1980's, 4 to 5 in every 10,000 children were thought to be afflicted.

Advertisement

The higher rate, described in today's issue of the Journal of the American Medical Association, is in line with rates found in recent smaller studies in the United States and abroad in which the autism prevalence was 4 to 6 children in 1,000.

The researchers, from the federal Centers for Disease Control and Prevention, said the prevalence rates they found would mean that at least 425,000 Americans under age 18 have some form of autism.

Dr. Marshalyn Yeargin-Allsop, an epidemiologist at the National Center on Birth Defects and Developmental Disabilities, led the study.

Some of the increase can be explained by changes in the definition of autism, a brain disorder in which normal social interaction is difficult or impossible. The definition has widened to include milder forms.

Many experts believe that autism results from an interplay of genes and unknown environmental factors.

Dr. Yeargin-Allsop said the researchers canvassed schools, clinics, doctors, nonprofit programs and other places that autistic children might have gone for services in 1996. Studies that look at autistic children in just one setting, like special clinics, tend to find lower rates, she said.

Experts reviewed the medical records of each child and determined whether autism was diagnosed accurately. They did not examine the children themselves. Out of the 289,456 children ages 3 to 10 living in the metropolitan Atlanta in 1996, 987 had mild to severe autism.

Dr. Yeargin-Allsop said 18 percent of the children found to have autism in 1996 had never had an accurate diagnosis. Many had been classified as having general developmental difficulties; the higher-functioning children had been missed entirely.

The Atlanta study found that rates were the same for blacks and whites but confirmed studies finding that autism is four times more common in boys than in girls.