Ruminant pestivirus infections of pigs have a worldwide distribution. The prevalence is varied and depends mainly on (i) contact with cattle, (ii) age of pigs and (iii) degree of homology of virus strains used for serology, with field strains of bovine virus diarrhoea virus (BVDV) infecting pigs. Emphasis should be laid on sources of BVDV other than cattle, e.g. contaminated vaccines and fetal calf serum. The need for differentiation of pestiviruses (hog cholera, bovine virus diarrhoea and Border disease viruses) is highlighted by the fact that clinical disease syndromes, e.g. growth retardation and wasting, are reminiscent of hog cholera. Monoclonal antibodies are available which differentiate between hog cholera virus (HCV) and ruminant pestiviruses, presumably BVDV. An up-to-date account of the antigenic relationship between pestiviruses is included in the review. Analysis of the in vitro host range of these viruses is considered to be important and may explain infections of pigs with pestiviruses other than HCV. Recent results have shown the existence of "specialists" amongst BVDV strains for bovine cells, and a few isolates also performed well in cultures of the PK15 cell line. In contrast, multipotent BVDV strains presumably have additional attachment sites for ovine and porcine cells. Identification of receptors on ovine and porcine cells could contribute to a clear distinction between BVDV and HCV infections of pigs. Immediate control measures for BVDV infections of pigs are not required. However, such infections may interfere with serologic surveys and surveillance on a herd basis and, therefore, impair eradication programmes and efforts to maintain the status in countries declared free of hog cholera.