A flock of sheep in which border disease (BD) was enzootic was studied through a breeding season. At the beginning of the study (August 1981), 125 (82%) of 152 ewes were seropositive to the cross-reacting bovine viral diarrhea virus. Within 7 months, 3 (18%) of 17 seropositive ewes retested had reverted to seronegative. Of the remaining 21 ewes identified as seronegative, 7 (33%) converted to seropositive by the end of the study. Triplet lambs were born, 2 of which exhibited clinical signs of BD. The virus was isolated from blood lymphocytes from both of the affected lambs. The most severely affected lamb shed virus into the urine, saliva, and feces through 10 weeks of age. Lymphocyte stimulation tests indicated that the lymphocytes from the affected lambs had decreased function in months 4 through 7, but returned to normal function by the eighth month. Transmission of BD virus was investigated by exposing 5 seronegative ewes to the BD-infected lambs. Two of the contact ewes developed viremia and 3 converted to seropositive within the 13-week exposure period. Evidence from this and other studies supports a model of BD in gravid, nongravid, and persistently infected adult sheep.