The attenuation of an avian pneumovirus (APV) isolate (APV/MN/turkey/1-a/97) by 63 serial passages in cell culture (seven in chicken embryo fibroblasts and 56 in Vero cells) and its evaluation as a live attenuated vaccine in turkey poults is described. The birds were vaccinated with two different doses of attenuated virus (104.5 median tissue culture infectious dose (TCID50)/ml and 102.5 TCID50/ml) at 2 weeks of age, and were challenged 2 weeks later with virulent APV. No clinical signs were seen in vaccinated, challenged birds, whereas severe clinical signs were observed in the mock-vaccinated, challenged group. Vaccinated birds developed anti-APV antibodies, which increased in titre following challenge with virulent virus. On challenge, none of the vaccinates was found to shed viral nucleic acid as detected by reverse transcriptase-polymerase chain reaction, but non-vaccinated, challenged birds did. The vaccine virus was also evaluated under field conditions in two farms. At one farm, the 'seeder bird approach' was used and two birds per 1000 birds were vaccinated by the oculo-nasal route. In the second farm, the virus was given to all birds simultaneously in the drinking water. The birds vaccinated by the drinking water route seroconverted earlier and continued to shed virus for longer as compared with birds inoculated by the seeder bird approach. The overall results of this study indicate that the 63rd passage of APV was sufficiently attenuated and offered protection against challenge with virulent virus.