Among non-primate vertebrates, feline immunodeficiency virus (FIV) infection in the cat may be the closest model of human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS). Clinical evolution and immunological and virological relationships between human HIV/AIDS and disease produced by FIV infection in cats are very close. These similarities should facilitate progress in the understanding of mechanisms of viral infection and immunopathology, and make this model potentially very valuable in evaluation of experimental therapeutic approaches to AIDS in man. Development of feline immunodeficiency virus vectors bearing therapeutic genes targeting different human diseases is a promising strategy for gene therapy, despite some recent studies which suggest that despite lack of evidence of infection of man by FIV, additional epidemiological surveillance may be indicated to determine if transmission can occur from this close companion to humans in some circumstances.