From the end of March to the beginning of December 1999, an epidemic of low-pathogenicity avian influenza (LPAI), caused by a H7N1 type A influenza virus, affected the intensively reared poultry population of Northeastern Italy. A total of 199 flocks were diagnosed with influenza infection. The highest number affected flocks were in meat turkeys (164), with only a limited number of turkey breeder, chicken (breeders, broilers, and table egg layers), and guinea fowl flocks infected. Following the circulation of the LPAI virus in a susceptible population for several months, a high-pathogenicity avian influenza (HPAI) virus emerged. Over 13 million birds on 413 different premises were affected by the HPAI virus, including turkey, chicken, guinea fowl, pheasant, Japanese quail, ostrich, and waterfowl flocks. In the present paper we report on the clinical, gross, histopathological, and immunohistochemical investigations performed on different avian species naturally infected by the LPAI virus and the HPAI virus.