We carried out a multicentre study in 36 hospitals from seven Spanish regions (Andalusia, Catalonia, Castile and Leon, Madrid, Navarre, the Basque Country and Valencia). Between July 2009 and February 2010 we selected hospitalized patients with influenza syndrome, acute respiratory infection, septic shock or multiple organ failure in whom influenza virus A (H1N1) 2009 infection was confirmed by real-time reversetranscription PCR (RT-PCR) from nasopharyngeal swabs; haemagglutinin (HA) sequencing was performed. We excluded patients who had nosocomial infection, defined as pandemic virus infection in a patient that appears ≥48 h after admission for another cause. All information collected was treated as confidential, in strict observance of legislation for observational studies. The study was approved by the Ethics Committees of the hospitals involved, following the Declaration of Helsinki principles. Written informed consent was obtained from all patients included in the study.