A severe course of a primary BVD infection in a seronegative dairy herd is reported. Approximately one third of the animals was very ill and showed severe diarrhoea, with temperatures varying from 39 degrees C to 41 degrees C. At the time of the outbreak, twenty-four cows had been pregnant for more than three months. Four animals aborted their calves, two mummies were born and two cows were found not to be pregnant at drying off during the next three months following the outbreak. In addition many of the newborn calves were weak and showed relatively low birth weights. The BVD virus was probably introduced into the herd by a purchased calf, which had pneumonia when it arrived on the farm.