BACKGROUND: Infants less than 6 months of age are at high risk for influenza disease and influenza-related complications, but no vaccine is licensed for this population. METHODS: A double-blind, randomized, placebo-controlled trial was conducted in 1375 healthy US infants 6 to 12 weeks of age. Subjects received 2 doses of trivalent inactivated influenza vaccine (TIV, Fluzone, sanofi pasteur; N = 915) or placebo (N = 460) 1 month apart in combination with indicated concomitant vaccines. Solicited adverse events were collected for 7 days following vaccination, and unsolicited adverse events for 28 days. Hemagglutination-inhibition antibodies to all 3 vaccine strains were measured following the second TIV/placebo dose. RESULTS: No significant differences were seen between TIV and placebo groups for any safety outcome. Fever > or =38 degrees C within 3 days of vaccination was seen in 11.2% versus 11.7% of TIV versus placebo recipients. Serious adverse events within 28 days were reported in 1.9% of TIV and 1.5% of placebo recipients. Antibody responses to childhood vaccines were similar in both groups. Increased influenza-specific antibody responses in TIV recipients compared with placebo recipients were seen against all 3 strains in TIV recipients (P < 0.001), with better responses to influenza A strains noted. Reciprocal geometrical mean titer to H1N1, H3N2, and B were 33, 95, and 11 in TIV recipients versus 7, 9, and 5 for placebo recipients. Over 90% of TIV recipients had antibody > or =1:40 for at least 1 vaccine strain and 49.6% for 2 strains, versus 16.4% and 0.9% in placebo-recipients. CONCLUSIONS: TIV administered to young infants beginning at 6 to 12 weeks of age is safe and immunogenic.