Ten outbreaks of calf respiratory disease in Danish dairy herds were investigated by lung lavage, and in eight of the herds paired blood samples were tested serologically. In six of the 10 herds bovine respiratory syncytial virus (BRSV) antigen was detected in the lung lavage fluids. In only one calf was coronavirus and BRSV detected simultaneously. The paired blood samples confirmed that four of the herds were BRSVinfected, and in one herd the BRSV infection was diagnosed by the paired blood samples alone. Significant increases in antibody titres against coronavirus were observed in two herds, both in combination with other virological agents. No adenovirus antigen was detected in any of the lavage samples, but in two herds a significant increase in antibody titres against adenovirus was observed. Parainfluenza-3 (PI-3) virus was not detected in the lung lavage fluids, and in four of the herds no antibodies to PI-3 were detected. In three herds no viral involvement could be found. The findings suggest that BRSV may be an important causative agent in calf respiratory disease in Denmark, even in very young calves.