question: INTRODUCTION: This study was undertaken to estimate the burden of morbidity associated with laboratory-confirmed influenza in children below 15 years of age. PATIENTS AND METHODS: Children presenting with acute respiratory infection and/or isolated fever at the Basurto University Hospital, Bilbao, Spain between November 2010 and May 2011 were included in this study (NCT01592799). Two nasopharyngeal secretion samples were taken from each; one for a rapid influenza diagnostic test in the emergency department, and the second for laboratory analysis using real-time polymerase chain reaction and viral culture. RESULTS: A total of 501 children were recruited, of whom 91 were hospitalized. Influenza diagnosis was confirmed in 131 children (26.1%); 120 of 410 (29.3%) treated as outpatients and 11 of 91 (12.1%) hospitalized children. A total of 370 of 501 children (73.9%) had no laboratory test positive for influenza. The proportion of subjects with other respiratory viruses was 145/501 (28.9%) cases and co-infection with the influenza virus plus another respiratory virus was detected in 7/501 (1.4%) cases. Influenza virus types were: A (H1N1 and H3N2) 53.2% (67/126); B (Victoria and Yamagata) 46.0% (58/126); A + B 0.8% (1/126). The median direct medical costs associated with each case of laboratory-confirmed influenza was €177.00 (N = 131). No significant differences were observed between the medical costs associated with influenza A and B. CONCLUSION: Almost half of the cases were influenza virus B type. The administration of a vaccine containing influenza A and B types to children below 15 years of age might reduce the overall burden of the illness.