The clinical consequences of infections with bovine virus diarrhoea virus (BVDV) were studied in a herd of dairy cattle, where BVDV circulated for approximately 2.5 years. Of the 136 cows that were subject to a primary infection, 129 remained healthy, 5 had mild signs, and 2 became severely ill; 1 of these 2 died from a concurrent puerperal infection. In spite of the predominantly subclinical infection, a gradual decrease of 10% or more in milk production, occurring within 10 days, was observed significantly more often in cows that seroconverted than in cows that did not seroconvert over the same period. Percentages of abortion, stillbirth, and birth of weak calves were not significantly higher in cattle that seroconverted during gestation than in cattle that did not seroconvert during gestation. Abnormal return oestruses after insemination, a possible sign of early embryonic death, and congenital abnormalities were not associated with the BVDV infection. In calves that had ingested colostrum from their seropositive dams, respiratory disease ran a significantly milder course than in calves from seronegative dams. The results indicate that, in addition to the known losses associated with the birth of persistently viraemic offspring, a 'subclinical' BVDV infection in a dairy herd may also result in substantial economic losses due to decreased milk yield and more severe respiratory disease in calves.