Since 1998, an epidemic of avian influenza has occurred in the Iranian poultry industry. The agent was pathotyped as non-highly pathogenic and subtyped as an H9N2 avian influenza virus. Therefore it did not require eradication. However, frequent incidences of high mortality were observed commonly on broiler farms. No other species of bird were affected. The circulation of the virus and mixed infection with other respiratory pathogens, particularly infectious bronchitis virus and Mycoplasma gallisepticum, were incriminated in the high mortality on poultry farms and resulting great economic losses. Clinical signs in both field and experimental studies included swelling of the periorbital tissues and sinuses, nasal and ocular discharge, and severe respiratory distress. However, in the experimental study, the mortality rate was much lower than in the natural outbreak. Gross lesions identified included extensive congestion of the respiratory tissues, and exudation with cast formation in the tracheal bifurcation, which extended to the secondary bronchi. Severe necrotizing tracheatis was the predominate histological lesion. Ultrastructurally, orthomyxovirus-like particles were identified in the inoculum used for the experimental study. An inactivated H9N2 avian influenza vaccine prevented mortality in experimentally challenged chickens.