In collaboration with infection control units and microbiology laboratories, we prospectively registered all proven and probable pediatric cases of influenza A/H1N1v infections hospitalized in four tertiary facilities of Brussels (Hôpital Universitaire des Enfants Reine Fabiola, Universitair Ziekenhuis of Brussels, Cliniques Universitaires Saint-Luc and Hôpital Saint-Pierre). These facilities totalize 406 pediatric beds, representing 80% of the total pediatric beds available in Brussels (about 1 million inhabitants in 2009). Moreover, three of the hospitals have a Pediatric Intensive Care Unit (PICU) where critically-ill children from other hospitals of Brussels and the surrounding areas are referred to (in total 32 PICU beds available).