After the introduction of pestivirus into a herd undergoing an embryo transfer and artificial insemination program, substantial post-weaning calf losses occurred. The predominant clinical feature was severe respiratory disease, in contrast to the commonly recognised mucosal disease. Thirty-one of 76 calves were affected, with a case fatality rate of 58%. All calves which were persistently infected with pestivirus died during the study period. There was a significant association in the surviving calves between the occurrence of recent pestivirus infection and respiratory disease. The losses on this property clearly indicate the need to routinely screen animals in an artificial breeding program for freedom from pestivirus infection.