Aetiology of endemic respiratory diseases was examined with bacteriological, virological and serological methods in Hungary. Several bacterium and virus species are included in the pathogenesis of endemic respiratory diseases of ruminants, however different environmental factors, nutritional and management deficiencies, overcrowding, ventilation problems and other stress factors predispose the animals to the disease. In the case of cattle parainfluenza-3 (PI-3) virus, adenoviruses and Mycoplasma bovis infection together with Mannheimia haemolytica serotypes A1 and A2, Pasteurella multocida serotype A and Histophilus somni proved to be the most important causative agents. Adenoviruses and PI-3 virus, serotypes A1, A2, A6, A8, A5 of M. haemolytica and P. multocida serotype A were most frequently detected in the aetiology of respiratory diseases of sheep. In the case of goats serotype A2 of M. haemolytica proved to be the dominating agent. Identification of the aetiologic agents of endemic respiratory diseases of ruminants is of special importance, since it is the basis of their control.