Five cases of leukemia believed to have occurred over a four-year period in the town of Saint-Philbert-en-Mauges (Maine-et-Loire) led the town’s mayor to file a report with the Maine-et-Loire Departmental Directorate for Social Affairs (DDASS). Following a preliminary study conducted by the Ddass, the Cire Ouest was consulted to confirm the report within a geographic area, defined a priori for the period 1997–2001, to identify and describe cases in this area by investigating common exposures that might be responsible for the occurrence of this cluster, and to assess the 20-year trend in leukemia mortality and all-cancer mortality in this area. (R.A.)