Respiratory disease caused by avian pneumovirus (APV) has a strong negative impact on the economy of the turkey industry in many countries. Progress in developing vaccines against this infection in the US has been slow partly because of the lack of a consistent challenge model to conduct vaccine efficacy studies. This study was designed to determine whether in vivo passages of a US isolate of APV, designated subtype C (APV-C), would increase virus virulence, leading to consistent clinical signs in turkeys. Three different experiments were performed. In experiments 1 and 2, a cell culture adapted APV was passaged four times in vivo in turkeys. Following each passage, clinical signs were found to increase in severity. In addition, inoculated birds were found to shed both APV RNA (by reverse transcriptase-polymerase chain reaction) and live virus (by virus isolation) at each passage. The mean antibody titres also increased with each passage. The results of the second experiment were not in complete agreement with those of experiment 1. In the third experiment, APV grown in three different cell lines was inoculated into three groups of turkeys. Clinical signs were observed in inoculated birds and virus could be isolated from all three groups. The results of this preliminary study indicate that in vivo passage of APV-C in birds may increase virus virulence, but the results obtained in experiment 2 suggest that further studies are needed to confirm this.