A case of diphtheria, which has not been seen in Norway for 30 years, was reported in 1992 in the northern part of the country bordering Russia. An increasing number of cases of diphtheria has been reported in the former USSR, including the northwestern part of Russia. In order to elucidate the potential of an epidemic spread across the Norwegian-Russian border, a seroepidemiological study was performed. A total of 470 sera, 243 from Finnmark, Norway, and 227 from Arkhangelsk, Russia, were examined for antibodies against diphtheria toxin, using an in vitro toxin neutralisation method. No statistically significant difference in the presence of diphtheria antitoxin between the Norwegian and the Russian populations was found. The presence of neutralising antibodies decreased by age, and this decrease was most pronounced among the Russians. Individuals aged 40 to 70 years, and Norwegian women in particular, seem to have an increased risk for diphtheria as judged by the diphtheria antitoxin levels.