The Chernobyl accident, which occurred on April 26, 1986, led to a surge in thyroid cancer cases in the most heavily exposed Eastern European countries, first observed in children in 1990 and now in young adults. In France, thyroid cancer is rare and has a good prognosis. However, the increase in these cancers over the past twenty years and the passage of the radioactive cloud over France in 1986 led the French Institute for Public Health Surveillance (InVS) to investigate the possible existence of a link between the Chernobyl accident and this increase. To address this question, since 2000 the InVS has been leading a national multidisciplinary initiative involving numerous stakeholders, notably the cancer registries grouped within the Francim network. Since 2003, the InVS has developed the surveillance system in accordance with the measures of the government’s cancer plan. It has strengthened its support for the registries and for the National Registry of Solid Tumors in Children, established in 2000. It is also developing a national multi-source system that complements and is closely integrated with the registry-based system, which will eventually monitor cancers across the entire country. On the occasion of the 20th anniversary of the Chernobyl accident, the InVS details the current results of epidemiological studies conducted with its partners and aimed at enhancing knowledge regarding the incidence and mortality of these cancers. Overall, the results do not point to a potential effect of the Chernobyl accident in France. The observed increase in thyroid cancer incidence rates is seen in most developed countries. Diagnostic techniques, as well as the more frequent performance of total thyroidectomy for benign conditions, lead to the incidental detection of cancers and likely play an important role in the observed increase. The significant regional disparities observed do not correspond to those of radioactive fallout from Chernobyl and may primarily reflect differences in medical practices. However, a high incidence is observed among men in Corsica. If future studies confirm an excess incidence among men, it will be necessary to clarify the reasons for this. In general, however, a better understanding of the risk factors for thyroid cancer in France remains necessary. To this end, the InVS and Inserm launched a call for projects in 2003. Four studies are currently underway, with results expected in 2008. (R.A.)