In January 2003, the InVS was commissioned by the Ministries of Health and Labor to investigate a possible cluster of kidney cancers at a chemical plant in the Allier department. A preliminary calculation estimated that, for male employees of this company between 1994 and 2002, the risk of kidney cancer was thirteen times higher than in the general French population. Given the significance of this risk and the possibility of an occupational origin, the InVS decided to launch an epidemiological investigation. The plant manufactures vitamins and amino acids intended for animal feed supplements. Among the many substances present at the plant, C5 chloroacetal—an intermediate in the new vitamin A synthesis process introduced in 1981—is suspected by some experts of being the cause of kidney cancer cases. The mortality study shows no significant excess of deaths from all causes or from cancer. An excess of deaths from kidney cancer is observed among women. However, the study of kidney cancer morbidity in the reconstituted cohort suggests an increased risk for the maintenance-utility and vitamin A production sectors using the new process. The case-control study, despite the small sample size, shows a statistically significant association between kidney cancer and exposure to C5 chloroacetal. Several factors support a causal link: a persistent association after adjusting for suspected risk factors; epidemiological findings consistent with toxicological test data. However, the causality of C5 chloroacetal could not be formally demonstrated due to difficulties in isolating exposure to this substance from that of other potentially mutagenic products; since the implicated manufacturing process is specific to the company, it is not possible to conduct a study on other populations. After 2003, new cases of kidney cancer continued to be reported among current and former employees of this plant. The InVS recommends continuing to monitor this population. (R.A.)