Surprisingly enough, there are still people promoting the idea that the rise in autism diagnoses observed over the last decades was caused by thimerosal in vaccines. The original argument was this–vaccines were added to the vaccine schedule in the 1990′s and with them the infant exposure to thimerosal increased. Concurrent with this rise in infant thimerosal exposure was a rise in autism diagnoses. Add to this a poorly concocted argument that autism resembles mercury intoxication and you have the basis for the mercury hypothesis.
Thimerosal was phased out of infant vaccines over 10 years ago. Thus, if the thimerosal hypothesis were true, reported autism rates should be declining by now.