Proventricular Dilatation Disease (PDD) is a very important letal disease in parrots. It affects several psittacine species and is a high risk factor for the health of breeding collections, but is rarely observed in other avian families. To date, the etiology of the disease remained unclear, though a virus infection was always assumed. Recently, a novel virus (Avian Bornavirus [ABV]) was discovered in parrots suffering from PDD so that ABV is now considered as the most likely cause. Despite the fact that clinically healthy birds may be infected with ABV, several studies demonstrate a correlation between ABV-infection and clinically present PDD. Apart from direct virus detection, serological methods allow the demonstration of an infection.Currently, Avian Bornavirus is the leading candidate as the aetiologic agent of PDD. Breeding collections and birds planned to be introduced into collections should therefore be investigated by molecular biological and serological methods for the presence of an ABV-infection. The diagnostic value of the demonstration of an ABV-infection for the diagnosis of a clinically present PDD has to be investigated further.