Bovine pestivirus infection has been mainly recognized as the cause of mucosal disease, a syndrome which occurs sporadically in weaned and adult cattle as a late sequel to foetal infection during the first 3-4 months of gestation. Infection has also been associated with the occurrence of congenital malformations, especially of the central nervous system. Following the development of improved diagnostic and research techniques, pestivirus has now been shown to be associated with significant early reproductive loss including fertilization failure, embryonic mortality and abortion. The principal determinant of the outcome of in utero infection in the bovine is the age of the conceptus when infection occurs.