Annual vaccination with trivalent influenza vaccine (TIV), and a single dose of tetanus toxoid-reduced diphtheria toxoid-acellular pertussis (Tdap) vaccine, are both recommended for adults in the US. This study was conducted to obtain information on the safety and immunogenicity of co-administered TIV and a Tdap vaccine (Boostrix) in US adults. The immunogenicity and safety of Tdap and TIV was evaluated in 1,497 adult subjects 19-64 years of age, who were randomized to receive Tdap and TIV either concomitantly or one month apart (TIV followed by Tdap). Seroprotection rates for diphtheria, tetanus and influenza antigens were high (>or=94.1%) for both vaccine regimens, and immune responses to these antigens in the concomitant group were non-inferior to those observed in the sequential group. Although antibody concentrations for pertussis antigens were lower in the concomitant group than in the sequential group, concomitant administration was shown to be non-inferior to sequential administration with respect to anti-pertussis toxoid concentrations one month after Tdap vaccination. For filamentous haemagglutinin and pertactin, the between-group differences in antibody concentrations marginally exceeded pre-specified limits for defining non-inferiority. In both groups, anti-pertussis antibody concentrations were greater than those observed in infants following primary DTaP vaccination, in whom vaccine efficacy against pertussis was demonstrated. Reporting of adverse events appeared to be similar between groups. The data support the conclusion that Tdap and TIV vaccines may be co-administered without compromising either the effectiveness or tolerability of either vaccine.