In the Netherlands, laboratory investigation was indicated for all hospitalised and/or deceased patients with suspected influenza A(H1N1) 2009 virus during the 2009-2010 pandemic as well as the 2010-2011 influenza season. Following laboratory confirmation of influenza A (H1N1) 2009 virus infection, name and clinical characteristics of hospitalised and deceased patients had to be reported to the municipal health service (MHS) by both the attending medical doctor and the head of the involved microbiology laboratory. The MHS entered the notifications into a national anonymous and passwordprotected web-based database, including structured questions about patient demographics and information on underlying medical conditions, treatments, clinical presentation, and admission to an intensive care unit (ICU). In the pandemic season 2009-2010, additional information on underlying conditions for deceased patients was collected by the Centre for Infectious Disease Control (CIb) of the National Institute for Public Health and the Environment (RIVM) in consultation with the MHS and subsequently added to the notification database.