Between November 1984 and February 1985, a serious outbreak of pyrexia, diarrhoea, agalactia, metritis and severe weight loss affected most of the recently calved cows in a 183-cow dairy herd in Norfolk. Fifteen cows died and 20 were culled. Forty cows aborted during or after the outbreak, and many of them produced mummified fetuses; 18 calves were stillborn and three others died soon after birth. Necropsy of three affected cows revealed ulceration of the gastrointestinal tract similar to that seen in cases of mucosal disease. Bovine virus diarrhoea virus was isolated from the intestines of one cow that died soon after the onset of illness. The virus was also isolated from the blood of four acutely ill cows and seroconversion was demonstrated in three of those that survived. The virus was isolated from three aborted fetuses, a stillborn calf and a live neonatal calf. Body fluids from two aborted fetuses were seropositive for the virus as were sera from all the aborting cows tested. In addition to widespread seroconversion to bovine virus diarrhoea virus during the outbreak, there was serological evidence of recent infection with Leptospira interrogans serovar hardjo and Coxiella burnetii in a high proportion of cows. It was concluded that this was primarily an acute outbreak of bovine virus diarrhoea but its unprecedented clinical severity was probably associated with the concurrent introduction of L hardjo and C burnetii into an immunologically naive herd during the main calving period. Epidemiological analysis suggested that the source of the virus and L hardjo was down-calving heifers returning from communal marsh grazing.