Introduction: The aim of the study was to evaluate the serological efficacy of Hiberix and Infanrix-DTPa vaccines in preterm infants. Materials and Methods: The results of the investigation of 61 preterm infants immunized three times (primary vaccination) with Hiberix and Infanrix-DTPa at 6-week intervals are presented. Of the 61 children, 17 were additionally immunized with a booster dose of these vaccines. Postvaccinal response to these immunizations was evaluated by an immunoenzymatic method. Results: We observed a significant increase in protective postvaccinal antibody titers against Haemophilus influenzae type b (Hib) and Bordetella pertussis after the primary vaccination compared with the initial antibody levels (p<0.05). A significant increase (p<0.0002) in protective antibody titers after the booster dose of Hiberix compared with the primary vaccination was also noted. No correlations between birth weight, gestational age, and the achieved levels of postvaccinal anti-polyribosylribitol phosphate of Hib and of anti-pertussis toxin and anti-filamentous hemagglutinin of B. pertussis antibodies after the primary vaccination or booster dose were found. After the booster dose, all the preterm infants responded with the production of protective postvaccinal antibody titers against Hib and B. pertussis. Conclusions: Due to the very good immunogenicity of the vaccines against Hib studied, inclusion of this immunization should be proposed in the obligatory vaccination schedule in Poland, especially in preterm infants. An additional immunization (i.e. a second booster dose) of Polish children with acellular pertussis (DTPa) vaccine is necessary to protect them from decreasing protective anti-pertussis antibody titers in early childhood.