The genomes of classical swine fever virus (CSFV) vaccine strains are poorly characterized, and the mechanisms for their attenuation remain unknown. The aim of the present study was to characterize the 3' noncoding region (3' NCR) of a number of attenuated vaccine strains of CSFV in order to examine changes in the viral genome after attenuation. The results showed that the 3' NCR:s of Porcivac, Rovac, Russian LK and original Chinese vaccine strain contain insertions very similar to that present in the published nucleotide sequence of the C-strain. Phylogenetic analysis showed that the vaccine strains Porcivac, Rovac and Russian LK vaccine were closely related to each other, but that these three strains showed a distant relationship with two tested variants of the Chinese vaccine strain (C-strain and original Chinese vaccine). The 3' NCR insertion is not likely to be a marker for attenuation of the virus, since many CSFV vaccine strains do not contain the insertion. The fact that the insertions occur in lapinized vaccine strains suggests that these genetic segments were incorporated during the adaptation of the virus to the rabbit host system.