In May 2002, a general practitioner in the town of Vauhallan (Essonne) reported to the DDASS the occurrence of two cases of leukemia, three months apart, in two children of the same age living in the same housing development. A study was initiated by the Cire IdF to investigate these cases, and a technical group comprising epidemiologists from the InVS, the Cire, the Inserm, and the IRSN, as well as a public health engineer from the DDASS 91, was established. (R.A.)