Since the 1980s a number of live attenuated type A and B Avian Pneumovirus (APV) vaccines have been developed in Europe and these have generally afforded good protection. All have been produced by repeated passages of field virus in a laboratory culture system to achieve various degrees of attenuation. They are widely used in commercial growing turkeys and to prime future layers and breeders. When tested under experimental conditions, these empirically derived vaccines were shown to be fully protective whilst not causing detectable disease themselves. However, they have not performed as well when used in the field and unstable attenuation has been considered to be a possible factor. This paper describes the first evidence of reversion of an APV vaccine in the field. First, systematic evidence of extended vaccine virus persistence in the field arose from longitudinal studies performed in the UK in 1995. Secondly, from an outbreak of Turkey Rhinotracheitis (TRT) in an Italian turkey flock previously vaccinated at day old, we isolated an APV which proved to be a vaccine derivative by sequencing. Finally, in order to determine whether vaccine virus or a derivative of increased virulence had been isolated, the virus was applied to 1 day old poults in secure isolation conditions. The vaccine derivative virus was shown to be able to cause disease with a severity similar to that caused by virulent field virus.