BACKGROUND: Influenza vaccination is recommended for the elderly in many countries, but immune responses are weaker compared to younger adults. OBJECTIVE: To investigate the impact of daily consumption of a probiotic dairy drink on the immune response to influenza vaccination in an elderly population of healthy volunteers over 70 years of age. DESIGN: Two randomised, multicentre, double-blind, controlled studies were conducted during two vaccination seasons in 2005-2006 (pilot) and 2006-2007 (confirmatory). Eighty-six and 222 elderly volunteers consumed either a fermented dairy drink, containing the probiotic strain Lactobacillus casei DN-114 001 and yoghurt ferments (Actimel, or a non-fermented control dairy product twice daily for a period of 7 weeks (pilot) or 13 weeks (confirmatory). Vaccination occurred after 4 weeks of product consumption. Geometric mean antibody titres (GMT) against the 3 viral strains composing the vaccine (H1N1, H3N2, and B) were measured at several time intervals post-vaccination by haemagglutination inhibition test. RESULTS: In the pilot study, the influenza-specific antibody titres increased after vaccination, being consistently higher in the probiotic product group compared to the control group under product consumption. Similarly, in the confirmatory study, titres against the B strain increased significantly more in the probiotic group than in the control group at 3, 6 and 9 weeks post-vaccination under product consumption (p=0.020). Significant differences in seroconversion between the groups by intended to treat analysis were still found 5 months after vaccination. Similar GMT results were observed for the H3N2 strain and H1N1 strain, confirming the results of the pilot study. CONCLUSION: These studies demonstrate that daily consumption of this particular probiotic product increased relevant specific antibody responses to influenza vaccination in individuals of over 70 years of age and may therefore provide a health benefit in this population.