The aim of this study was to evaluate diphtheria immunity in a sample of the Turkish population having high childhood immunization coverage, including a booster dose of diphtheria toxoid at 12-15 years of age. A total of 599 persons aged 1-70 years were selected with cluster sampling. The information on socio-demographic characteristics, vaccination status and diphtheria history was gathered for each participant. Diphtheria antitoxin levels were measured qualitatively by using micro-enzyme immune assay. Of studied population, 72.3% had fully protective antitoxin levels (>= 0.1 IU/ml). The rate of protection was 92.5% in the children aged 0-2 years, 93.2% in the primary school children aged 7-9 years, and 86.0% in the adolescents aged 15-19 years. After 20 years of age, diphtheria protection rates showed a significant age-related decrease, reaching minimum in the 30-39 age group, in which 47.3% of these subjects had fully protective antitoxin levels. The diphtheria antitoxin geometric mean titer (GMT) was highest in the 0-2 year age group (1.18 IU/ml). In the adolescents aged 15-19 years, diphtheria antitoxin GMT was 0.71 IU/ml. Then, geometric mean titer decreased with increasing age, and reached the minimum level in the 40-59 years age group (0.18 IU/ml). The protection rate among females was significantly lower than males (67.1% vs. 80.9%). The difference was apparent in the 20-29 and the 30-39 years age group: 80% of the males and 46.2% of the females in the 20-29 years age group, and 60% of males and 44.1% of females in the 30-39 years age group were fully protected against diphtheria (p < 0.0001). These results suggest that in Izmir. Turkey, full serological protection against diphtheria is only detectable in <50% of the young adult population, even though childhood immunization coverage is relatively high. Potentially, there is still risk of diphtheria outbreaks among the adults in our country. Therefore, a revaccination of adults with reduced doses of diphtheria toxoid should be considered to sustain diphtheria immunity. (C) 2011 Elsevier Ltd. All rights reserved.