University of California, Davis researchers followed more than 600 infants. About 19 percent of those who had one older sibling with autism ended up with the same diagnosis by 3 years old.
Previous smaller or less diverse studies reported a prevalence of between 3 percent and 14 percent.
"We were all a bit surprised and taken aback about how high it is," said lead author Sally Ozonoff, a with the Mind Institute at UCD.
Researchers also found that 32 percent of those with at least two older siblings with autism went on to also develop the disorder.
Among boys with autistic siblings- 26 percent developed autism versus 9 percent of girls. Autism is already known to be more common in boys.
Autism has no known cause, but doctors believe that genetics and external influences are factors. The study is online in the Journal of Pediatrics.
The Associated Press contributed to this story.
