During 1993 outbreaks of diarrhoea in adult dairy cows in three geographically unrelated herds were found to be caused by bovine viral diarrhoea virus (BVDV). The affected animals showed signs of acute watery diarrhoea, agalactia and pyrexia (39.4 to 42 degrees C). Ulceration of the buccal mucosa, a mucoid nasal discharge and stiffness were inconsistent signs. The disease spread rapidly in each case. The diagnosis was confirmed by the isolation of non-cytopathic BVDV from blood and tissues and by the demonstration of significantly rising titres to BVDV by an ELISA. The highest morbidity recorded was 40 per cent with one herd experiencing a 10 per cent mortality. There was no increased incidence of abortion in any of the herds, either at the time of or subsequent to the outbreaks of diarrhoea. In one herd the purchase of a persistently viraemic heifer 14 days before the outbreak was thought to be the source of infection, but in the other two herds the source was not established.