To define the characteristics of malignancy we performed routine histology and an immunohistochemical study on seventeen aortic body tumors in dogs. We essayed tumors using a panel of immunohistochemical markers: neuron specific enolase (NSE), chromogranin A (CrA) and S-100. Among 17 cases, the neoplastic cells were positive for NSE (17 cases, 100%), S-100 (9 cases, 53%), and CrA (8 cases, 47%), respectively. The sustentacular cells density and chief cell staining intensity were both inversely related to tumor grade. The most relevant data was consistent with a negative staining of S-100 correlated with absence or decreased number of sustentacular cells in tumors grade III. This report indicates that the immunohistochemical panel has utility for the diagnosis of chemodectoma and the negative staining to CrA and S-100 markers in tumors grade III expresses an indication of malignant behaviour of the tumor.