Twelve herds were investigated where outbreaks of clinical Aujeszky's disease had occurred. Clinical Aujeszky's disease was eliminated from all farms following vaccination. After vaccination was stopped in two of the six fattening herds virus was also apparently eradicated, judging from serological examination. These two herds were the smallest of the six fattening farms and size and the throughput of pigs may have contributed to apparent eradication of virus. In two of six breeding farms where controlled vaccination is still practised unvaccinated gilts and boars were seronegative. In this case possible eradication of infection may have resulted from either no excretion or insufficient production of virus from vaccinates to infect susceptible non-vaccinates. It is postulated that a properly controlled and monitored vaccination and culling programme may result in the eradication of disease and perhaps also infection from breeding herds experiencing Aujeszky's disease.