Abstract: An outbreak of tumors diseases occurred in Shandong free range chickens on a commercial farm of Shandong Province in China during September 2008. The flock containing 6 000 chickens, and the symptoms were first seen at 80dayold. The mortality reached 15% at 120dayold. The main symptoms were characterized by emaciation, ruffled feathers, paralysis and visceral tumors. Some birds showed unilateral limb paralysis and skin tumors. 66 serum and feather samples were collected from the sick chickens to evaluate the presence of antibodies to MDV, ALVJ and REV using agar diffusion reaction and enzymelinked immunosorbent assay (ELISA) technique. The results of agar diffusion reaction showed that the MDV antigen positive rate was 59.09%, and MDV antibodies positive rate was 65.16%, antigenantibodypositive rate was 37.88%. The overall seroprevalence of ALVJ antibodies was 7.58%, while the antibodies to REV were negative. Numerous similar nodular lesions and resembling tumors in one or more tissues were seen at necropsy. Some organs showed intumescence. Histopathological findings revealed large islands of leukomonocytes or myelocytes in tissues containing tumors. Immunostaining for MDV, ALVJ and REV monoclonal antibody revealed a diffuse presence of MDV and ALVJ antigen. PCR analysis revealed the presence of MDV and ALVJ proviral sequences in DNA of affected tissues. All the findings observed in this case were considered to be related with the MDV and ALVJ mixed infection. Prevention and decontamination to these diseases in local free range chickens should be noticed.