Avian adenoviruses (Aviadenovirus) belong to three groups. Group I contains five aviadenovirus species (A-E) with twelve serotypes isolated from fowls as well as duck adenovirus, pigeon adenovirus and turkey adenovirus. Group I of aviadenovirus plays no major role in mixed infections, although FadV-4 strains cause a dangerous disease in chickens - hydropericardium-hepatitis syndrome. Group II of the aviadenovirus includes the hemorrhagic enteritis virus of turkeys, marble spleen disease virus of pheasants and the splenomegaly adenovirus of chickens. In fact, the adenoviruses of this group are included in the Siadenovirus genus because the techniques of molecular hybridizations and DNA sequencing have indicated that there is a gene coding for sialidaze in their genome. The egg drop syndrome virus however, belongs to group III. Atadenovirus has been proposed as a name for this group of aviadenoviruses as it reflects the high adenine-thymidine (AT) content in their genome. The significance of adenovirus in pathology within poultry is constantly increasing.