Chromosomal aberrations were observed after vaccination against classical swine fever (CSF) in a previous study done on experimental pigs. To determine if the same effect occurs in farm animals, field trials were done with immunized pigs. The cytogenetic analysis was made from lymphocyte cultures of pigs sampled in three farms of Río Cuarto region on six different periods: one, pre-vaccination (control) and in five post-vaccination (days 3, 7, 10, 15 and 22). Vaccine inoculation induced significant increase of cell frequency with chromosomal aberrations from days 3 to 10, when maximal frequencies of 6.78, 10.36 and 7.21% were observed in farms A, B and C, respectively. Afterwards, a decrease was recorded, reaching values not significantly different from controls. Mean frequencies of cells with chromosomal aberrations were 2.8+/-0.3, 5.9+/-0.4, 3.1+/-0.3%, for A, B and C farms, respectively. Cytogenetic effect was clastogenic, being chromosome breaks the first type of alteration to increase from the day 3 post-vaccination onwards, followed by chromatid exchanges and cells with multiple abnormalities. Chromosome pulverization was the last type to show increment, reaching a top value on day 10, after that it started to diminish gradually. There was no difference in the frequency of polyploid cells among sampling dates meaning that this type of alteration would not be induced by vaccination. Results of this field study confirm the mutagenic capacity of live virus vaccines against CSF and indicate that the evolution of chromosome alterations on the five evaluated post-vaccination periods was similar at different environments.