The possibility that autism is linked to traffic pollution has been raised by researchers in California.
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.
Data from the US Environmental Protection Agency were used to work out levels of pollution for addresses in California.
This was used to compare exposure to pollution, in the womb and during the first year of life, in 279 children with autism and 245 without.
The researchers from the University of Southern California said children in homes exposed to the most pollution "were three times as likely to have autism compared with children residing in homes with the lowest levels of exposure".
They have previously shown a link between autism and living close to major roads.
They warn that there could be "large" implications because air pollution is "common and may have lasting neurological effects".