Objective. We performed a phase II randomized, controlled, open-label, single-center study (Centros de Estudios de Infectología Pediátrica, Colombia) to examine the feasibility of combined administration of seasonal and MF59-adjuvanted A/H5N1 influenza vaccines using extemporaneous mixing or simultaneous administration. Methods. The primary objective of the study was to assess the immunogenicity of seasonal influenza and A/H5N1 vaccines using European licensure criteria (Committee for Medicinal Products for Human Use [CHMP]); the secondary objective was to assess vaccine reactogenicity and safety. Results. In 401 healthy 18–40-year-old subjects, both vaccines were immunogenic in all settings; the vaccine for seasonal influenza met all CHMP criteria, unaffected by coadministration of A/H5N1 vaccine in separate or mixed injections. Likewise, the immunogenicity of A/H5N1 vaccine was unaffected by seasonal influenza vaccination, with hemagglutination inhibition seroprotection rates of 28%–40% after 1 dose and 67%–80% after 2 doses, sufficient to meet CHMP criteria. Solicited local and systemic adverse events were mainly mild to moderate. No vaccine-related serious adverse events were reported during the study period. Conclusions. These data demonstrate that seasonal and MF59-adjuvanted A/H5N1 influenza vaccines can be given as a mixed injection or by simultaneous separate injections without affecting immunogenicity or safety, supporting the feasibility of incorporating prepandemic MF59-adjuvanted A/H5N1 vaccines into seasonal influenza vaccination programs and the development of tetravalent influenza vaccines, including pandemic strains. Clinical Trials Registration. NCT00481065.