Over a period of 1 year a seroepidemiological study was conducted at the outpatient clinic of a trauma department. Immunity to diphtheria was determined in serum samples from 558 injured patients (205 women and 353 men, age from 18 to 70). Diphtheria-antitoxin concentrations were measured with an enzyme immunoassay and a tissue culture toxin-neutralization assay. Sero-immunity was classified as susceptibility (<0.01 IU/ml), basic protection (0.01 - <0.1 IU/ml) and full protection (≥0.1 IU/ml) against the toxic manifestations of the disease. A total of 27.1% of the subjects were susceptible to diphtheria, 26.5% had basic protection, and 46.4% were fully protected. The median antitoxin concentration reached 0.08 IU/ml (0.0-0.29; quartiles Q25-Q75). A non linear trend toward decreasing immunity with increasing age was observed (P < 0.001) and females proved less protected than males (P = 0.006). The country of original immunization (Austria, Western European countries, Eastern European countries and Non European countries) had no influence on sero-immunity (P = 0.49). Multiple linear regression analysis revealed that age (P < 0.001) and gender (P = 0.004) had a significant independent influence on diphtheria immunity level, whereas the country of original immunization was not significant (P = 0.72).