Thiomersal, also called thimerosal, is an ethyl mercury derivative used as a preservative to prevent bacterial contamination of multidose vaccine vials after they have been opened. Exposure to low doses of thiomersal has essentially been associated with hypersensitivity reactions. Nevertheless there is no evidence that allergy to thiomersal could be induced by thiomersal-containing vaccines. Allergy to thiomersal is usually of delayed-hypersensitivity type, but its detection through cutaneous tests is not very reliable. Hypersensitivity to thiomersal is not considered as a contraindication to the use of thiomersal-containing vaccines. In 1999 in the USA, thiomersal was present in approximately 30 different childhood vaccines, whereas there were only 2 in France. Although there were no evidence of neurological toxicity in infants related to the use of thiomersal-containing vaccines, the FDA considered that the cumulative dose of mercury received by young infants following vaccination was high enough (although lower than the FDA threshold for methyl mercury) to request vaccine manufacturers to remove thiomersal from vaccine formulations. Since 2002, all childhood vaccines used in Europe and the USA are thiomersal-free or contain only minute amounts of thiomersal. Recently published studies have shown that the mercury levels in the blood, faeces and urine of children who had received thiomersal-containing vaccines were much lower than those accepted by the American Environmental Protection Agency. It has also been demonstrated that the elimination of mercury in children was much faster than what was expected on the basis of studies conducted with methyl mercury originating from food. Recently, the hypothesis that mercury contained in vaccines could be the cause of autism and other neurological developmental disorders created a new debate in the medical community and the general public. To date, none of the epidemiological studies conducted in Europe and elsewhere support this assumption. Although any effort should be made to avoid useless exposure of vaccinees to a potentially toxic compound, it should be emphasized that 1) public communication on this issue has led to a decrease in the hepatitis B vaccination coverage of children born to HBs Ag positive mothers in the US; 2) this issue was not really relevant in France where until 2002, apart from two hepatitis B vaccines, all childhood vaccines were thiomersal-free, and 3) in developing countries using multidose vaccine vials, moving to thiomersal-free vaccines in unidose presentations would represent such an incremental cost that millions of children would no more have access to vaccination. Therefore the World Health Organisation still recommends the use of thiomersal-containing vaccines as part of the expanded programme of immunisation.