Equine Adenovirus 1 (EAdV1) is primarily an upper respiratory tract infection of horses often leading to bronchopneumonia which can be fatal particularly in foals. The prevalence of EAdV1 in southern NSW was determined by an indirect ELISA, the antibody prevalence was 54.9% positive which is consistent with a survey conducted in New Zealand to be at 52.5% (Dunowska, et al. 2002). The results of this study confirm that this virus is widely prevalent in the horse population tested and possibly in other horse populations as well. Recently we sequenced and characterised the EAdV1 genome and found the characteristic traits of a Mastadenovirus with genes including; protein V and IX along with the E1A, E1B, E3 and E4 regions all present. The EAdV1 genome had a high degree of similarity to Canine Adenovirus (CAdV1 or 2) and the Bat Adenovirus TJM (BtAdV-TJM) and it is believed that EAdV1 may share a common ancestor to the CAdV1 and 2 and BtAdV-TJM. Phylogenetic analysis further supports the co-evolution of these viruses.