Avian reticuloendotheliosis (RE), is a neoplastic disease of poultry caused by reticuloendotheliosis virus. The occurrence of RE in both chickens and turkeys has an immunosuppressive effect and may lead to vaccination failures. Avian reticuloendotheliosis virus (REV), is widely distributed in different kinds of birds, causing subclinical infections. Another important issue adhering to this disease is contamination of vaccines against fowl pox (FP) and Marek’s disease (MD) with REV. Standard criteria used for diagnosis include history, clinical signs, gross necropsy, and histopathology. There are no efficient vaccines against RE nor treatment. A rare neoplastic disease of turkeys known as lymphoproliferative disease (LPDV), that has been reported in USA, Europe and Israel, is induced by type-C retrovirus associated with tumors in a number of organs in domestic turkeys (Meleagris gallopavo).