Sinus adenocarcinomas of papillary pattern are neoplasms rarely described in dogs and most cases are characterized by slow growth, local invasion and rare cases of metastasis. Their characteristics are presence of glandular structures that may have papillary pattern, tubulo-papillary, acinar and mixed pattern. Approximately 80% of primary nasal tumors are malignant and adenocarcinomas are the most frequently histology type. This paper reports a case of papillary adenocarcinoma of nasal sinus in the dog, emphasizing the use of histochemical and immunohistochemical techniques to aiding for appropriate therapy. Case: A 14-year-old male Poodle dog, presenting seropurulent nasal discharge, frequent sneezing and an enlargement in the nasal region, was submitted to surgical excision of a tumor. Macroscopically, was observed fragment coated with soft hairy skin and shear blackish color, and some fragments associated with more fi rm tissues like cartilage and bone tissue. Tumor specimens were collected, fi xed in 10% neutral buffered formalin solution and embedded in paraffi n. Afterwards, 4 μm histological sections were obtained and stained with HE. For immunohistochemical analysis a biotin-peroxidase system was used and secondary antibodies were identifi ed using Advance HRP. CKAE1/AE3, CK34βE12, SMA, E-cadherin, Cox-2, Ki-67 and Her-2 expression were evaluated. For CK AE1AE3 antibody a qualitative method was used and neoplasia were classifi ed as negative (-) or positive (+) and for CK34βE12 was used semiquantitative evaluation and neoplasms were classifi ed as negative (-), positive with focal staining (+) and positive with diffuse staining (++). The graduation for HER-2 expression was (+) for 4 (++++), were 4 (++++) was the most intensity membrane staining. For Cox-2 the system used was semiquantitative and the distribution was scored by estimating the percentage of tumor cells staining. Intensity was estimated on a scale from 0 (absent) to 12 (strong). The proliferative index was calculated by counting nuclei positive for Ki-67 (anti-MIB-1) in a total of 1000 neoplastic cells. Microscopically, was observed a proliferation of epithelial cells in papillary arrangement with delicate connective axis of support and areas of stromal invasion, and the tumor received the diagnosis of papillary adenocarcinoma of nasal sinus. Immunohistochemical analysis revealed strong staining for Cox-2 (score 12), CKAE1/AE3 positivity and CK34βE12 (+ + +) and negative for Her-2 (+). Loss of E-cadherin expression was observed and SMA allowed the visualization of areas of basal membrane rupture. The rate of proliferation was evaluated by means of the expression of MIB-1 was high (16%). Chemotherapy was performed with carboplatin and gemcitabine.