Almost half of children with autism in a new study had run away at least once - and many of them were missing long enough to cause concern.
Researchers found that kids most often wandered off from their home, school or a store, and some tried to run away multiple times a day.
But rather than being confused about where they were, kids typically left to find a place they enjoyed, to explore or to avoid an anxious or uncomfortable situation, based on their parents' reports.
"It's rooted in the very nature of autism itself," said Dr. Paul Law, who worked on the study.
"Kids don't have the social skills to check in with their parents, and to have that communication and social bond that most children have when they're approaching a road or at a park."
Law is the director of the Interactive Autism Network Project at the Kennedy Krieger Institute in Baltimore. With funding from a number of autism research and advocacy groups, he and his colleagues used their registry to survey the parents of 1,218 kids with an autism spectrum disorder.
Of those kids, 598 - or 49 percent - had tried to run away at least once, their parents reported. And 316 were missing long enough to cause concern - an average of more than 40 minutes.
In comparison, the same parents reported 13 percent of their non-autistic children had ever wandered off after age four.
Most of the kids with autism who went missing were in danger of getting hit by cars, and others could have drowned. Police had to be called for one-third of missing children.
"Amongst the families we did interview, there were many reports of injuries, close calls with drowning (and) close calls with traffic accidents," Law told Reuters Health.
"There's an enormous burden that all families are undergoing to keep their families safe. The amount of diligence, and not going out in public, and staying up late at night… just the general anxiety that families live under because of concerns with this is just torturous."
Children with more severe autism were more likely to have bolted, according to findings published Monday in the journal Pediatrics.
Autism researcher Russell Lang from Texas State University-San Marcos said the prevalence of running away or "eloping" in children with autism "absolutely surprised" him.
"It's a very dangerous behavior, and it's a little bit deceptive because it can seem somewhat benign compared to other challenging behaviors," Lang, who wasn't involved in the new study, told Reuters Health.
Those other "challenging behaviors" common in kids with autism include self-injury and property destruction, he said. They often get lumped together with running away, which is why researchers haven't had a good estimate of the prevalence of elopement, itself, until now.
The number of children diagnosed with an autism spectrum disorder, which includes autism and Asperger's syndrome, has increased in recent years. The Centers for Disease Control and Prevention now estimate that one in 88 children has a diagnosis of one of the conditions.
The new study couldn't estimate how many children with autism die every year due to running away and getting into danger, the researchers said.
"This is not simply a case of parents being remiss in some way regarding their supervision of their children," Lang said. "The child with autism doesn't realize what danger they're putting themselves in. They have a propensity to elope, it seems, regardless of parental care."
He said therapy that rewards kids for not wandering off may help prevent them from disappearing in the future.
Law said parents can reach out to advocacy groups to learn about safe locks for their doors and tracking devices for kids. And emergency responders can be better prepared for getting the call when a child with autism goes missing.
Still, he added, "we haven't totally come to consensus on what some of the best practices are" to prevent running away.