Infection of pet cats by FeLV results in virus replication in cells of the immune system. The virus causes both immune cell depletion and dysfunction which result in several FeLV-induced immunopathologic diseases and numerous secondary immunosuppressive diseases which together are the major cause of death of FeLV-infected cats. FeLV infection also activates complement and causes hypocomplementemia. In addition, persistent FeLV infection results in the formation of intact FeLV and FeLV structural antigen-antibody immune complexes which cause immune complex glomerulonephritis. Recently it has been possible to reverse FeLV viremia and, in some cases, cause regression of lymphosarcoma by removing FeLV circulating immune complexes from the plasma. This latest advance in experimental therapy together with our current understanding of the immunopathologic effects of FeLV gives hope that, someday, we will learn how to overcome the deleterious effects of the virus. Such a success with FeLV-infected cats would, hopefully, be applicable to retrovirus or non-oncogenic viral-induced immunopathologic diseases of humans.