In data taken from the US state of Missouri, influenza A predominated during the 2004–2005 season and affected all age groups. The highest incidence of laboratory-confirmed influenza (43%) was reported among adults aged 15–64 years. The groups with the next highest percentage of laboratory-confirmed influenza (18% each) were those aged 5–14 years and >65 years. The lowest percentage (9%) was reported among those aged 2–4 years.