In November 2005, the mayor of the town of Ruitz (population 1,588) reported to the Pas-de-Calais Departmental Health and Social Affairs Office (DDASS) that two cases of brain cancer had been diagnosed in children attending the town’s preschool, which enrolls approximately 95 children each year. One of the children died in 2004; the second case was diagnosed in November 2005. Near this school stands a tower topped with a base station (an SFR mobile phone relay), which has been in operation since early October 1998. Parents are concerned about the antenna’s impact on their children’s health. Parents’ concerns regarding the operation of this mobile phone antenna have existed for several years. As early as 1999, local residents and parents mobilized to have the tower relocated. They sought assistance from organizations including Priartem (an association advocating for regulations on the installation of mobile phone antennas). The investigation conducted by Cire Nord and presented in this report follows the initial steps of the InVS Methodological Guide for the Assessment and Management of Spatio-Temporal Clusters of Non-Infectious Diseases: - verification of the health signal through the collection of medical data on the cases and the search for any other cases of pediatric cancers among children who attended the school; - literature review on childhood cancers and their established or suspected risk factors; - literature review on mobile phone relay antennas and the occurrence of health effects; - measurement of electromagnetic field levels emitted by the cell tower located near the Ruitz preschool (reactivated for one day), followed by comparison with reference levels. This measurement was conducted by a certified engineering firm, in the presence of a notary and several parents of students; - search for data on other potential sources of environmental exposure in the municipality (in particular, a study of industries in the industrial zone). The hypothesis of a link between the occurrence of pediatric cancers and exposure to radiofrequency electromagnetic fields (RF EMF) emitted by the relay antenna cannot be upheld within the scope of this investigation. Indeed, the main scientific uncertainties regarding the potential health effects of RF EMFs relate to cell phone use (risk of tumors resulting from concentrated exposure to the head and neck) and not to exposure to cell towers. Furthermore, electromagnetic field measurements taken in Ruitz confirmed low exposure levels, well below regulatory limits. At the conclusion of the investigations, we were unable to identify any significant overexposure of the Ruitz population to an environmental risk factor. It is possible that the cluster of cancers observed in children is due to individual risk factors and the random distribution of cancers across the region. Statistically, it can be estimated that if all groups of 300 children in the area were monitored for 10 years, there would be at least 400 instances where, purely by chance, three cancers would be observed. In a situation such as that in Ruitz, where the number of cases is small and involves different types of cancer, it is proposed to continue epidemiological surveillance of the possible occurrence of other pediatric cancers in the municipality by regularly querying the two national pediatric cancer registries (R.A.)