A suspected cluster of breast cancer cases among staff at an administrative building of a research institute in Meurthe-et-Moselle was investigated by Santé publique France and its regional intervention unit (Cire) in Lorraine, in collaboration with members of the Gast (Occupational Health Alert Group) and the relevant occupational health service. For the 10 reported cases, the investigation focused on i) case identification, ii) reconstructing career paths within the company and occupational and environmental exposures, iii) the medical description of each case, iv) a literature review of the epidemiological characteristics of breast cancer, v) the identification of known or suspected risk factors for these cancers, vi) an assessment of the carcinogenic potential of the products and equipment used. Diagnosed between 2003 and 2013, the cases affected nearly one in five women who had worked in the construction industry between 1969 and 2013, suggesting an excess risk of developing this cancer compared to data from the general French population. Each case presented between 1 and 5 individual risk factors. Investigations did not identify ionizing radiation, electromagnetic fields, a pyralene transformer, or chemical compounds. For all these factors, either the observed concentrations were below regulatory limits, or it was not possible to estimate past exposures (particularly to chemical compounds). Santé publique France and the GAST proposed the following to the company: raising staff awareness about organized breast cancer screening, addressing psychosocial risks, and continuing efforts to prevent chemical risks.