Nicholas Kristof reports growing evidence that there is a connection:

Concern about toxins in the environment used to be a fringe view. But alarm has moved into the medical mainstream. Toxicologists, endocrinologists and oncologists seem to be the most concerned.
One uncertainty is to what extent the reported increases in autism simply reflect a more common diagnosis of what might previously have been called mental retardation. There are genetic components to autism (identical twins are more likely to share autism than fraternal twins), but genetics explains only about one-quarter of autism cases.
Suspicions of toxins arise partly because studies have found that disproportionate shares of children develop autism after they are exposed in the womb to medications such as thalidomide (a sedative), misoprostol (ulcer medicine) and valproic acid (anticonvulsant). Of children born to women who took valproic acid early in pregnancy, 11 percent were autistic. In each case, fetuses seem most vulnerable to these drugs in the first trimester of pregnancy, sometimes just a few weeks after conception.

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Frankly, these are difficult issues for journalists to write about. Evidence is technical, fragmentary and conflicting, and there’s a danger of sensationalizing risks. Publicity about fears that vaccinations cause autism — a theory that has now been discredited — perhaps had the catastrophic consequence of lowering vaccination rates in America.
On the other hand, in the case of great health dangers of modern times — mercury, lead, tobacco, asbestos — journalists were too slow to blow the whistle. In public health, we in the press have more often been lap dogs than watchdogs.
At a time when many Americans still use plastic containers to microwave food, in ways that make toxicologists blanch, we need accelerated research, regulation and consumer protection.
“There are diseases that are increasing in the population that we have no known cause for,” said Alan M. Goldberg, a professor of toxicology at the Bloomberg School of Public Health at Johns Hopkins University. “Breast cancer, prostate cancer, autism are three examples. The potential is for these diseases to be on the rise because of chemicals in the environment.”

A physician I know in Dallas who specializes in treating children on the autism spectrum once told me, “This is a disease of industrialism.” He said at the time (two years ago) that there was as yet no conclusive evidence linking autism to environmental toxins, but he was confident that it would emerge as a major contributing factor.