Lesions of chickens inoculated with two highly pathogenic avian influenza virus strains, A/turkey/England/50-92/91 (H5N1) and A/chicken/Victoria/1/85 (H7N7) were examined histologically and immunohistochemically. Birds of both treatment groups died within 5 days post-inoculation. The most significant lesions induced by these two viruses consisted of swelling of the microvascular endothelium, systemic congestion, multifocal haemorrhages, perivascular mononuclear cell infiltration, and thrombosis associated with viral antigen in the vascular endothelium and/or perivascular parenchymatous cells. Viral antigen in the cardiac myocytes was consistently detected in all birds. In addition, severe pulmonary congestion and oedema was found in A/turkey/England/50-92/91 virus-inoculated birds that died within 1 day post-inoculation. The other chickens of both groups showed necrotic and inflammatory changes associated with viral antigen in various organs and tissues. These findings suggested that cardiovascular system involvement played an important role in the pathogenesis of these virus infections.