Six pediatric cases of bone marrow aplasia (BMA) involving children residing in Ille-et-Vilaine and treated in 2004 by the pediatric hematology department at the Rennes University Hospital (compared to the usual rate of one case per year) led to a report being filed with the Ille-et-Vilaine Departmental Directorate for Social Affairs (DDASS). Following a preliminary study conducted by the Ddass, the Cire Ouest was notified to confirm the report and undertake an investigation aimed at describing the cases by searching for common exposures that might be responsible for this cluster. Given the highly specialized care required for this condition, a search for additional cases was conducted in other university hospitals in Brittany as well as in four Parisian referral centers. Information on potential exposures of the cases was collected through hospital physicians in contact with the children’s families. Age-standardized incidence ratios (SIR) were used to compare the number of observed cases with the number of cases that would have been expected if the incidence of the disease had been that of the French population. The incidence in the department of Ille-et-Vilaine was higher than expected (SIR = 33, p<0.001). No common exposure to the cases was identified. Surveillance has been implemented in Ille-et-Vilaine and neighboring departments. The objective of the surveillance is to monitor changes in the incidence of AM in Ille-et-Vilaine over time in order to objectively determine either a return to normal or a sustained high incidence, which would lead to further etiological investigations. (R.A.)