BACKGROUND: The incidence of diphtheria has been markedly reduced and almostly eradicated by widespread use of DTP vaccines in developed countries. However, outbreaks of this disease may be occurred under some circumstances of ineffective immunization. In recent time, some studies reported persistent outbreaks of diphtheria in developed countries and indicated the existence of a large pool of susceptible individuals with potential for epidemic infection. In Korea, diphtheria vaccination has been well maintained since 1956 with high acceptant vaccination rates. So, there has been no reported diphtheria patient since 1987. But, there has been few study to diphtheria serosuvey, and no assessment of diphtheria immunization. Also, we do not use Td vaccine in adults for maintenance of diphtheria immunity. In this aspect, we conducted age related seroepidemiology of diphtheria and indirectly assessed the immunity of diphtheria vaccines, used in Korea. METHODS: For the evaluation of age related serosurvey of diphtheria immunity in Korean populations, study subjects below 10 years old aged children were classified into 10 groups (A~J) with one year interval, and beyond this age to 60 years old aged adults were classified into 5 group (K~O) with 10 years interval. And the adults over 60 years old age was classified into the last group (P). The numbers of each group were 100, and sex distributions of each group were almostly equal. And for the indirect assessment of diphtheria immunization in Korean children, children under 15 years old were classified into 6 groups (I~ VI) according to the status of DTaP vaccination. The numbers of this each group were 30, and sex ratio was almostly equal. Detection of specific IgG antibody to diphtheria toxin were determined by ELISA (contained fragment A & B toxin; IBL, Germany). RESULTS: In age related groups, the antibody titers to diphtheria toxin were well maintained until 10 years old age group, thereafter the titers abruptly decreased below 0.1 IU/mL and then slightly elevated after 30 years old age group and then maintained with low levels. In the groups related DTaP vaccine status, the antibody titers were very low (below 0.07 IU/mL) in prevaccination status, but the titers after primary vaccinations were markedly increased and maintained (above 0.6 IU/mL) until 15 years. And diphtheria antitoxin levels in the groups (L, M, N) showed no significant differences between males and females. CONCLUSION: The results of our study showed that the antibody titers to diphtheria toxin in the 20~50 years old aged groups dramatically decreased. This result indicated that vaccine induced diphtheria immunity did not last throughout life, and Td vaccination program in adults should be considered for maintenance of diphtheria immunity. And the immunity to diphtheria in Korean children indicated that 3 timesprimary and 2 times booster diphtheria immunizations were optimal.