A large number of immigrants have come to Greece from diphtheria-endemic countries during the past 8 years. Information on the immune status against diphtheria among immigrants from Albania and the New Independent States (NIS) of the former USSR, as well as the Greek population, is essential in planning public health control measures. The main objective of the study was to determine the risk of the spread of diphtheria if new cases arrived in the country. Diphtheria antitoxin (DAT) in sera from 185 healthy individuals from northern Greece and 227 immigrants from Albania and the NIS was titrated using a quantitative ELISA test. The participants were adults aged 22 to 46 years. Titers were considered as protective when DAT levels were higher or equal to 0.1 IU/ml. Albanian immigrants exhibited the highest rates of protective titers (99 of 167, 59.3%), while lower immunity rates were detected among immigrants from the NIS (20 of 60, 33.3%) (1.5 < OR = 2.9 < 5.4, p = 0.0007) as well as among Greek participants (47 of 185, 25.4%) (2.8 < OR = 4.4 < 7.0, p = 0.00001). Immunity rates did not differ between Greek and NIS participants. DAT levels of persons with protective titers differed between the three population groups (analysis of variance, p = 0.0005), with the highest mean DAT values detected among NIS immigrants (0.613 IU/ml). Lower values were found among protected Albanians (0.482 IU/ml) and Greeks (0.314 IU/ml). There was no significant age- and sex-related difference in immunity rates and DAT levels among the groups. The low immunity rates among Greek adults might suggest that they have had no natural contact with toxigenic strains of corynebacteria. A booster dose of diphtheria toxoid for local adults is recommended, to reduce their risk of acquiring toxigenic strains from individuals who may carry the bacteria without exhibiting clinical disease.