In the present report, the extent of the reduction in Aujeszky's disease virus (ADV) dissemination achieved when pigs were intensively vaccinated with gI-deleted vaccines under field circumstances, was examined. On widely dispersed breeding-fattening farms, a gI-negative status was most rapidly obtained and the rate of new waves of infections was lowest when the attenuated Bartha strain was administered to both the sows and the fatteners. It was more difficult not only to reach but also to keep a gI-negative status on farms on which the sows were vaccinated with an inactivated vaccine and the fatteners with the attenuated Bartha strain or when the fattening pigs were not vaccinated at all. In a densely populated area, 9 of the 17 farms had gI-positive fatteners at the start of the intensive vaccination programme in which the attenuated Bartha strain was given to both the sows and the fatteners. Antibodies were not detected in the sera of the fatteners of each farm at some time during the experiments, but the fatteners on 7 of the 18 farms still showed antibodies against gI after 20 months of vaccination. At the end of the experiment, the percentage of fatteners with antibodies on these farms was markedly reduced compared with the percentage at the start of the experiment. Therefore, elimination of field virus may be feasible if intensive vaccination is carried out over a sufficiently long period of time. However, the high rate of reinfections experienced either due to reintroduction of the virus or to recrudescence should be a warning against too much optimism, particularly in regions with a dense swine population.