PURPOSE: The universal vaccination against hepatitis B during early infancy is the only effective way to control hepatitis B infection in highly endemic areas in Korea. The purpose of this study was to assess the effectiveness of simultaneous hepatitis B vaccination with DPT and oral polio at 2, 4, 6 months of age in babies of HBs antigen negative mothers. METHODS: One hundred and eighty-one infants who were admitted to the Neonatal Intensive Care Unit (NICU), Chonnam University Hospital from August, 1994 to December, 1995 were enrolled in this study. Infants received Hepavax-B 0.5ml (10 microgram) at contralateral thigh intramuscularly, simultaneously with DPT vaccination. Antibody was assessed by MEIA (microparticle enzyme immunoassay) at seven to nine months of age. RESULTS: The seroconversion rate of Hepatitis B after vaccination was 98.3%. There were no significant differences in positive rate of antibody according to gestational age, birth weight and sex. Seroconversion rate to hepatitis B in twins was lower than in singlets. Twenty-two infants among 23 infants, who were able to assess antibody titer, showed effective antibody titer, above 100mIU/ml. CONCLUSION: The results suggest that the simultaneous administration of hepatitis B vaccine with DPT/TOPV is very effective and simplifies schedule. Moreover, this schedule will improve the compliance of vaccination.