In January 2003, the French Institute for Public Health Surveillance (InVS) was asked by the Ministries of Health and Labor to investigate a possible excess of kidney cancer cases at a chemical plant in the Allier department. The alert had been brought to the attention of the relevant ministries by the company’s labor unions. Ten cases were the basis for the report, nine of which were diagnosed following an annual abdominal ultrasound screening program established in 1986 to detect liver angiosarcomas among workers exposed to vinyl chloride monomer (VCM). Following this report, a meeting initiated by the InVS was organized in February 2003 with company management, the occupational physician, employee representatives, the regional medical inspector, and the Auvergne branch of the Pension and Occupational Health Insurance Fund (Carsat). This meeting provided an opportunity to exchange information and gather additional details regarding the history of the problem, the description of the observed cancer cases, the company’s workforce, production processes, and the products used, as well as the preventive measures implemented within the company. The company in question manufactured vitamins A and E and amino acids. To this end, nearly 600 substances were or had been used, including C5 chloroacetal, an intermediate in the vitamin A synthesis process introduced in 1981. However, some experts suspected this substance of being the cause of the observed cases of kidney cancer. In vitro studies had demonstrated the mutagenic potential of this product in the early 1990s. At the same time, measures to prevent exposure to this product had been implemented, and workers exposed to C5 chloracetal had subsequently benefited from the abdominal ultrasound screening program. Several steps were taken by the InVS: . verification of the validity of the reported cases. The ten cases of kidney cancer that prompted the report had been histologically confirmed. All were adenocarcinomas; . verification of the excess incidence of kidney cancer. A calculation estimated that, for men employed at this company between 1994 and 2002, the risk of kidney cancer was 13 times higher than in the general French population. Given the significance of this risk and the plausibility of an occupational origin, the InVS decided to launch a two-part epidemiological investigation: a retrospective cohort study and a case-control study within the cohort. In parallel with the epidemiological investigations, the plant commissioned the INRS to conduct a study of chemical exposure risks related to vitamin A production and toxicological studies on C5 chloracetal. (R.A.)