Researchers in Los Angeles looked at 304 children with autism and nearly 300 normally developing children.
They found those whose moms were living within 1,000 feet of a freeway when they gave birth had an increased risk for autism.
But they are also stressing the study does not prove exposure to air pollution or traffic causes autism.
What they do suggest is that certain environmental exposures along with genetics and more could be part of the puzzle.
"It seems that air pollution could potentially affect the brain and be related to autism," said Dr. Heather Volk, Children's Hospital Los Angeles. "There's some nice data coming out of animal studies that seem to indicate that exposure to air pollution is associated with changes in the brain."
The full report can be found online in the journal Environmental Health Perspectives.
According to the Centers for Disease Control and Prevention, between 2002 and 2006 the number of reported cases of autism rose by 57 percent.