Research was undertaken to study the effect of a 'three times a year vaccination schedule in sows' against Aujeszky's disease (AD) on the maternal immunity of their offspring at a large pig farm. Only gI-negative sows and piglets were induced in the study. The maternal immunity was measured in the 8-week-old offspring of 6 gilts and 13 sows, vaccinated in early pregnancy and in 8-week-old piglets born from 12 gilts and 21 sows vaccinated 3 weeks prior to farrowing. At an age of 8 weeks only 5 percent of the piglets, born from sows vaccinated during early pregnancy showed antibody titres against AD of > 1:40 in their serum. In piglets born from sows vaccinated 3 weeks prior to farrowing more than 50 (54.4) percent of the piglets showed antibody titres > 1:40 in their serum. It was concluded that using a 'three times a year vaccination schedule' in a gI negative herd may lead to groups of seronegative piglets on breeding and multiplier farms. The implications are discussed.