Mercury and mercurial compounds are among the environmentally ubiquitous substances most toxic to both wildlife and humans. Once released into the environment from both natural and anthropogenic sources, mercury exists mainly as three different molecular species: elemental, inorganic, and organic. Potential health risks have been reported from exposure to all forms; however, of particular concern for human exposure relate to the potent neurotoxic effects of methylmercury (MeHg), especially for the developing nervous system. The general population is primarily exposed to MeHg by seafood consumption. In addition, some pharmaceuticals, including vaccines, have been, and some continue to be, a ubiquitous source of exposure to mercurials. A significant controversy has been whether the vaccine preservative ethylmercury thiosalicylate, commonly known as thimerosal, could cause the development of autism. In this review, we have discussed the hypothesis that exposure to thimerosal during childhood may be a primary cause of autism. The conclusion is that there are no reliable data indicating that administration of vaccines containing thimerosal is a primary cause of autism. However, one cannot rule out the possibility that the individual gene profile and/or gene-environment interactions may play a role in modulating the response to acquired risk by modifying the individual susceptibility.