On April 11, 2019, the occupational physician at a company specializing in digital services reported to the Île-de-France Regional Health Agency (ARS) that several cases of glioma had occurred since 2015 among employees at one of the company’s sites located in the Yvelines department (78). He indicated that he suspected exposure to electromagnetic fields might have played a role in the occurrence of these cases. Following this report, on July 8, 2019, Santé publique France received a joint referral from the ARS and the Île-de-France Regional Directorate for Enterprises, Competition, Consumer Affairs, Labor, and Employment (Direccte) to conduct an epidemiological investigation. The agency thus conducted an investigation to identify and document the cases, objectively assess the excess of cases, and determine whether or not there were exposures that could be linked to this excess, if applicable. The investigative approach adopted is based on methods developed by Santé publique France for the assessment and management of spatio-temporal clusters of non-infectious diseases. This process follows a multi-step decision tree. Based on the health and environmental data collected at each stage, a decision is then made on whether to continue the investigation. In total, seven cases of neuroepithelial tumors diagnosed between 2016 and 2019 were identified among employees at the company’s site. Calculations to assess the excess of cases showed that the number of neuroepithelial tumor cases was significantly higher than expected given national incidence data and the demographic structure of the studied population. This confirmed the existence of an excess of cases. Furthermore, the environmental investigations conducted and the available scientific data did not identify, within the employees’ occupational environment, any exposure that could potentially explain the excess cases and for which a more in-depth epidemiological study might be warranted. A number of questions have emerged in light of the information gathered. Consequently, Santé publique France deemed it appropriate, beyond the response to the referral from the ARS and the Direccte, to supplement the investigative work with additional information. This information involves: 1) a review of the medical records of the identified cases by a team of experts; and 2) a comparison of the questions raised by this investigation with the latest findings regarding the link between radiofrequency exposure and cancer, which is being conducted concurrently by ANSES. The systematic literature review conducted by ANSES as part of its expert assessment concluded that the totality of available data (epidemiological, animal, and mechanistic) “does not allow for a conclusion regarding the existence or absence of an effect of radiofrequency radiation on the development of brain and central nervous system tumors.” The review of medical records by neurologists concluded that the 7 cases presented characteristics (age at initial diagnosis, anatomical location, prognosis, microscopic features, immunohistochemical markers, molecular data) similar to those most frequently observed in the general population and corresponding to IDH-normal glioblastoma, grade 4 according to the 2021 WHO classification. Overall, the investigations confirmed the higher-than-expected number of cases, the absence of new cases since 2019, and the absence of current overexposure to radiofrequencies or electromagnetic fields, and did not identify a common cause for these cases. A link was established with ANSES’s most recent work on risks associated with radiofrequencies. The investigation into this cluster is closed and may be reopened should new cases arise.