Their study of more than 500 children said those exposed to high levels of pollution were three times more likely to have autism than children who grew up with cleaner air.
However, other researchers said traffic was a "very unlikely" and unconvincing explanation for autism.
The findings were presented in the Archives of General Psychiatry journal.
However, other researchers questioned how pollution could alter the brain's development and lead to autism.
Uta Frith, a professor of cognitive development at University College London, said: "It seems to me very unlikely that the association is causal."
USC/CHLA research shows autism risk for children exposed to air pollution during pregnancy, first year of life
Edited by Render, 28 November 2012 - 09:34 AM.