The current situation of bovine virus diarrhoea-mucosal disease (BVD-MD) virus infections is briefly reviewed, with special reference to problems arising from fetal infection in the Hokkaido district, a northern island of Japan. The results of some investigations on BVD-MD virus carried out in Japan are also described. Although bovine congenital anomalies caused by BVD-MD virus and mucosal disease may be late sequelae to fetal infection and occur sporadically, it appears that their incidence and importance in the Hokkaido district have increased in recent years. The results of antigenic characterisation of the recent isolates and serologic survey on bovine sera suggested that BVD-MD viruses with various antigenic properties are widespread among cattle in the district. There is also a possibility that clinical manifestations in infected cattle may differ with the antigenicity of the viruses. In addition, the virological investigations on experimental and naturally occurring mucosal disease suggested that persistently infected cattle are populations at high risk of developing mucosal disease. The antigenic homology of non-cytopathogenic persistent virus and cytopathogenic virus is probably an important factor in the pathogenesis of mucosal disease.