Researchers at the University of California at Davis followed 664 infants in the U.S. and Canada who had at least one older sibling with autism. About 19 percent, or 132 infants, ended up with an autism diagnosis as well by their third birthday.
Lead researcher Sally Ozonoff, who's with the UC Davis Mind Institute, says "We were all a bit surprised and taken aback about how high it is."
The highest rates were in infants who had at least two older siblings with autism - that was 32 percent. Also, boys with autistic older siblings had a 26 percent chance of being diagnosed themselves versus 9 percent of girls. Autism is known to be more common in boys.
The study is in Monday's Pediatrics online.