Prospective, active surveillance to capture severe influenza-associated complications and fatalities was performed from October 2005 to May 2008, covering three influenza seasons. Patients fulfilling the study case definition were included through the Surveillance Unit for Rare Paediatric Diseases in Germany (ESPED), a nation-wide voluntary reporting system covering 448 paediatric hospitals and departments. These hospitals contain approximately 21,500 beds, including about 2,650 paediatric intensive care beds (National Hospital Statistics, 2008). ESPED actively collected monthly reports on the occurrence of severe influenza-associated cases requiring intensive care treatment from all hospitals. Monthly reporting was requested even if no cases were identified. For identified cases, the hospitals were asked to return anonymous, structured questionnaires on basic demographic characteristics, underlying chronic medical conditions, influenza vaccination status, and influenza diagnostics, symptoms, treatment, pre-defined complications, and sequelae. The reporting physician filled in the questionnaire according to the information from the medical chart. If feasible, hospitals provided additionally anonymous medical discharge letters or autopsy results.