Three outbreaks of mucosal disease were investigated. Careful examination of 47 cattle that were persistently viraemic with bovine virus diarrhoea virus (BVDV) revealed no clinical disease, no or low levels of BVDV antibody and only non-cytopathic virus in their blood. The four animals with mucosal disease all showed clinical disease and both cytopathic and non-cytopathic virus in their blood. Following post mortem examination, there were particularly high levels of cytopathic virus in gut tissue. A hypothesis for the induction of mucosal disease is suggested. It states that animals become persistently infected with non-cytopathic virus following in utero infection and when, in post natal life, they become superinfected with a cytopathic virus, then mucosal disease ensues. The experimental reproduction of mucosal disease in support of this hypothesis is described.