An intensive vaccination programme with the glycoprotein I (gI) and thymidine kinase-deleted vaccine strain 783 was applied on all the pig farms in a region with a high pig density. To monitor the spread of Aujeszky's disease virus within breeding herds in that region, all the breeding stock in nine herds were examined for antibodies to gI six times at intervals of four months. The prevalence of gI-seropositive sows decreased greatly in all nine herds. The mean percentage of gI-seropositive sows decreased from 56.4 per cent (range 80.0 to 13.6 per cent) at the start, to 20.3 per cent (range 29.1 to 1.6 per cent) after two years. Nevertheless, seroconversions to gI were detected in all the herds, and in six out of the nine breeding herds even during the second year of the study. The intensive regional vaccination apparently did not completely prevent Aujeszky's disease virus infections within these herds. The source of the virus responsible for these infections was not identified. However, because in most herds only a few sows seroconverted, the virus either circulated at a low level within the herds, or its introduction or reactivation did not lead to an extensive spread of the virus.