The younger brothers and sisters of children with autism appear to be at greater risk of developing the condition than had been thought, according to research released Monday.

Based on earlier studies, researchers had estimated that between 3 percent and 10 percent of children whose older siblings were autistic were diagnosed with the disorder.

But in the new analysis, published in the journal Pediatrics, researchers calculated that the proportion is actually higher — closer to 19 percent overall. In families with more than one child with autism, the risk was even greater. about 32 percent of children with more than one older autistic sibling was diagnosed with the condition.

“Parents often ask what their risk of having another child with [autism] is, and until now we were really not sure of the answer,” said Sally Ozonoff of the University of California at Davis.

In the largest study of its kind, Ozonoff and her colleagues followed 664 infants with at least one older brother or sister who had been diagnosed with autism.

By age 3, the researchers found, 132 of the younger children, or almost 19 percent, had autism, a complicated and sometimes devastating disorder that affects a child’s ability to think, communicate, interact socially and learn. As expected, the risk varied by sex. More than 26 percent of the boys were diagnosed with autism, compared with 9 percent of the girls. Because milder forms of autism, such as Asperger disorder, are often diagnosed later in life, the risk may “in fact be higher than that reported here,” the researchers wrote.

“This study shows that the younger siblings of children with autism spectrum disorders need to be tracked very carefully, and this may require more than the normal surveillance that a pediatrician might typically do,” Ozonoff said.