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In 1996, a combined vaccine against both hepatitis A and B was licensed and commercialized and has been recommended for healthcare personnel in Belgium. This study compares the immunogenicity against hepatitis B virus (HBV) and safety of two vaccination schedules (0-1-12 months and 0-1-6 months) with this vaccine. This is a randomized, stratified and controlled study in healthy adult workers, who are not occupationally exposed to HBV. Seroconversion (≥1 IU/L) and seroprotection (≥10 IU/L) rates were compared using Fisher's exact test; geometric mean concentrations (GMCs) of anti-HBs were compared using one-way ANOVA. All statistical analyses were carried out with SPSS 11 on Apple Macintosh. A total of 399 subjects were enrolled in the study, and 356 were analysed according to the protocol. The rate of ≥10 IU/L at 6 months was 70.6% in the group 0-1-12 and 79.9% in the group 0-1-6; this rate decreased to 55.9% at 12 months in the first group. Seroconversion and seroprotective rates against HBV measured at month 13 in group 0-1-12 (98.9% and 95.6%) and measured at month 7 in group 0-1-6 (99.4% and 97.1%) were not statistically significantly different. GMC of anti-HBs after the 0-1-12 schedule was more than two fold higher than after 0-1-6 schedule ( P < 0.001). Reported side effects were comparable in both groups with a slight tendency to fewer side effects in the 0-1-12 group after the third dose. The results from our study show that the completed schedule 0-1-12 offers at least equal protective immunogenicity against HBV as the completed 0-1-6 schedule. People not receiving their third dose at 6 months can be given this dose up to 12 months after the first dose. The drawback of this flexibility, however, is the longer time period before the protection becomes effective. [ABSTRACT FROM AUTHOR]