The public immunization program against diphtheria, established in 1941, has almost eradicated the disease in Denmark, and 1956 became the first year without any notified cases. Since then, toxigenic strains have only been isolated five times--three cases of clinical diphtheria due to Corynebacterium diphtheriae biovar. mitis and two cases of tonsillitis/pharyngitis due to Corynebacterium ulcerans. The source of the infection was not identified in any of the cases. The first case of diphtheria in 1968 was imported from abroad. The following two cases in 1983 and 1985 were due to strains of the same phage type and peptide profile as the strains isolated during the epidemic in Sweden in 1984-1986. This indicates that the Danish cases and the Swedish epidemic derived from the same source. The diphtheria immunity of the Danish population is decreasing, and the level of protection is approaching the Swedish level. The impact is that a situation like that in Sweden may be anticipated with diphtheria epidemic in the lowest socio-economical groups--the skid row dwellers, alcoholics and drug abusers--if the immunization program against diphtheria is not intensified.