BACKGROUND: This study (NCT00383123) compared the immunogenicity and safety of 2 trivalent inactivated influenza vaccines: Fluarix [GlaxoSmithKline (study vaccine)] and Fluzone [Sanofi Pasteur (control vaccine)] in children 6 months to <18 years. METHODS: Children, stratified by age and randomized, received either study (N = 2115) or control vaccine (N = 1210) at day 0 (and day 28 for previously unvaccinated children younger than 9 years). Children 6 months to <5 years comprised the according-to-protocol (ATP) cohort for immunogenicity, whereas the reactogenicity/safety group included all children 6 months to <18 years. The study aimed to demonstrate immunologic noninferiority of study vaccine versus control vaccine. RESULTS: For children 6 months to <5 years, the predefined noninferiority criteria were not reached, mainly due to the differences in immune response in children 6 months to <3 years with no influenza vaccination history. All reactogenicity/safety endpoints were within the same range in both vaccine groups. CONCLUSIONS: The study vaccine demonstrated a good safety and reactogenicity profile; however, it did not meet the predefined noninferiority criteria in children 6 months to <5 years. The study vaccine was as immunogenic as the control vaccine in children aged 3 to <5 years.