In the fourth quarter of 2001, community organizations in the Epi d'Or neighborhood of Saint-Cyr-l'Ecole reported to various institutions the occurrence of various health conditions among the population, including three cases of childhood cancer, which they attributed to the presence of cell phone towers on the roof of a school. The Directorate General for Health then referred the matter to the French Institute for Public Health Surveillance (InVS) in April 2002 to investigate the situation. Based on current scientific knowledge (low power of the base station, multiple sources of exposure), the InVS determined that there was no justification for studying a link between cell phone towers and the onset of diseases. However, it was appropriate to assess whether the reported health concerns constituted a cluster. The investigation was entrusted to the Cire Ile-de-France in collaboration with the Yvelines Departmental Health and Social Affairs Directorate (DDASS) and with methodological support from the InVS. Subsequently, school physicians interviewed about the reported issues identified 8 cases of cancer among schoolchildren in the municipality between 1991 and 2002. Since the diversity of the initially reported conditions and symptoms did not support the existence of an aggregate, the investigation focused solely on cases of childhood cancer across the entire municipality. The objective of the investigation was to verify the existence of a cluster of childhood cancers (ages 0–14) in the municipality of Saint-Cyr-L’Ecole between 1990 and 2002 and to identify a possible environmental source that could be responsible. (R.A.)