Following a report by the occupational health service regarding what was perceived as an abnormally high number of cancer cases in a building housing two research laboratories at the University of Rennes 1, Cire Ouest launched an epidemiological investigation. The objective was to validate the report and identify common characteristics among the cases that might explain this cluster. Eight cases of breast cancer, three cases of non-Hodgkin lymphoma, and one case of chronic lymphocytic leukemia occurred over an eight-year period among the 97 women in the building—representing, respectively, seven times more breast cancers and 22 times more non-Hodgkin lymphomas than expected in the general population. A search for known risk factors for the reported conditions was conducted. A list of all products potentially handled, along with an assessment of their carcinogenicity, was compiled. The cases’ handling of or exposure to these products was documented. None of the women with cancer had handled or been aware of exposure to known carcinogens for their specific cancer. The cumulative doses of ionizing radiation for women who wore a dosimeter were lower than the average annual dose of natural radioactivity in France. Before considering the reconstruction of a cohort of all individuals who worked in this building, its feasibility must be explored and its objectives clarified. The recommendations issued aim to ensure safe workplace practices, establish a system to reconstruct employees’ exposure histories in the future, and continue to identify and document new cases of cancer among these staff members. (R.A.)