OBJECTIVE: Previous studies have reported that the immune response of hepatitis B vaccine is decreased in preterm or low birth weight babies compared to healthy term babies. To evaluate this notion, we investigated the positive seroconversion rate of hepatitis B antibody after hepatitis B vaccination according to gestational age and birth weight. METHODS: We retrospectively studied medical records about the positive seroconversion rate of a total of 968 infants who were given DNA recombinant hepatitis B vaccine as scheduled at Il Sin Christian Hospital, Busan from January 2002 to Mar 2007. RESULTS: The postive seroconversion failure rate (non-responder or poor-responder) of hepatitis B antibody by gestational age was 15.6% (<28 weeks), 11.4% (>28 weeks, <32 weeks), 5.8% (>32 weeks, <37 weeks), and 1.9% (<37 weeks). The postive seroconversion failure rate by birth weight was 16.7% (< 1,000 g), 15% (>1,000 g, <1,500 g), 5% (>1,500 g, <2,000 g), and 2% (>2,000 g). CONCLUSION: The seropositive rate of hepatitis B antibody after hepatitis B vaccination decreased as the gestational age and the birth weight decreased. And gestational age more than 37 weeks and birth weight above 2,000 g were independently associated with a higher seropositive rate of hepatitis B antibody after hepatitis B vaccination.