This report presents the results of the initial work carried out by the Working Group, established and coordinated by the InVS at the request of the Directorate General for Health, which was tasked with assessing the appropriateness and feasibility of establishing a national surveillance system for thyroid cancer. This report reviews the risk factors for this cancer. It provides an overview of the sources and tools available for recording new cases. It constitutes an essential step in analyzing the observed increase in the incidence of thyroid cancer in France and its temporal and geographical variations. It presents the epidemiological studies undertaken to support the Group’s hypothesis that variations in screening practices (ultrasound, fine-needle aspiration), diagnostic practices (anatomopathological procedures), and treatment practices (surgical procedures) for thyroid conditions contribute to the increased incidence of this cancer. This report will be followed by a final report to be released in 2002, which will propose the Group’s recommendations for strengthening the epidemiological surveillance system for thyroid cancers and will present the results of the studies on practices. (R.A.)