purpose: Currently available hepatitis B vaccines are recombinant, yeast-derived preparations given in 10-μg or 20-μg doses. The optimum dose remains controversial. We sought to assess the relative immunogenicity of two hepatitis B vaccines, given in different doses, in older individuals. patients and methods: In a multicenter, double-blind, randomized clinical trial, a total of 460 healthy subjects between 39 and 70 years of age were screened and immunized with either Engerix-B 20 μg or Recombivax HB 10 μg in standard, intramuscular, 3-dose regimens. Of these, 397 subjects were eligible to continue vaccination. Immunogenicity was measured by determination of antibody to hepatitis B surface antigen (anti-HBs). Seroconversion and seroprotection rates, and geometric mean titers of anti-HBs were calculated at 1, 3, 6, and 8 months after the initial dose of vaccine. results: Seroprotection rates for subjects receiving the 20-μg dose of vaccine were slightly, but not significantly, greater than for subjects receiving the 10-μg dose, at each time point. However, at 3 months, males receiving the higher dose had significantly higher seroprotection rates than males receiving the lower dose: 63% versus 37% (p <0.001). At 8 months, geometric mean titers for the group receiving Engerix-B 20 μg were significantly greater than that for the group receiving Recombivax HB 10 μg: 840 mIU/mL versus 340 mIU/mL (p = 0.001). conclusions: Immunization with the 20-μg dose of recombinant hepatitis B virus vaccine appeared to result in more rapid development of seroprotective anti-HBs titers in older men and in higher titers of anti-HBs at the completion of vaccination when compared to the 10-μg dose. The latter data suggest that the 20-μg dose may result in a longer duration of seroprotective anti-HBs titers.