A serological survey to determine the immunity to diphtheria in the Flemish population was conducted according to the recommendations of the World Health Organization. Immunity to diphtheria was determined on a randomised, stratified sample (1679 serum samples) from an existing serum bank (4058 serum samples) representative of the Flemish population. All age groups between 0 and 100 years were included. A tissue (Vero cell) culture toxin neutralisation assay was used to measure serum diph-theria antitoxin concentrations. The results showed that 43% of the Flemish population was protected against diphtheria (antitoxin titre, > or = 0.1 IU/ml), while 32% was susceptible (antitoxin titre, < 0.01 IU/ml); for 25%, protection was of limited duration (antitoxin titre, > or = 0.01 IU/ml and < 0.1 IU/ml). The proportion of susceptible subjects showed a significant age-related increase, with the highest values in the 35 to 44 and 45 to 54 age groups (57.9% and 55.5%, respectively). These results emphasise the need for booster immunization of adults.