ABSTRACT. Calazans S.G., Fonseca-Alves C.E., Rodrigues P.C. & Magalhães G.M. [Canine cutaneous mast cell tumor, with progression of low-grade to high-grade - Case report.] Mastocitoma cutâneo canino, com progressão de baixo grau para alto grau - Relato de caso. Revista Brasileira de Medicina Veterinária, 38(2):147-152, 2016. Curso de Medicina Veterinária, Universidade de Franca, Av. Dr. Armando Salles Oliveira, 201, Parque Universitário, Franca, SP 14404-600, Brasil. E-mail: email@example.com Mast cell tumor is a common canine cutaneous neoplasm with variable biological behavior. Many studies in the literature evaluate predictive and prognostic factors for this neoplasm. The tumoral progression is an important phenomenon for many neoplasms, and in the canine prostatic neoplasm and squamous cell carcinoma this progression is well defined, however, don’t have a classic describe in the cutaneous canine mast cell tumors. This study reports the diagnostic and treatment assessment of cutaneous mast cell tumor in a dog presenting tumor progression. A five years old Shar Pei dog, male, 20 kg, was admitted to the veterinary clinic presenting chronic alopecia and pruritus and cutaneous nodules in left pelvic limb. The tumor was surgically excised and histopathological and immunohistochemistry examination diagnosed a low grade of mast cell tumor, membrane localization of c-KIT (KIT I) and ki-67 under 23 cells, in five high magnification fields. The tumor recurred five months after initial diagnostic presenting a nodule in the surgical region. After cytological examination, mast cell tumor was confirmed and, cytoreductive chemotherapy was started. The patient presented no response to treatment and new nodules arrised, so surgical resection was performed. Histopathological and immunohistochemistry examination was performed again demonstrating tumor progression as high grade mast cell tumor, citoplasmatic localization in c-KIT (KIT III) and ki-67 superior 23 cells. After surgical procedure, tyrosine kinase inhibitors therapy was started, but twenty days after the patient presented adverse effects and died. The mast cell tumors are common in Shar Pei dogs, by which histopathological and immunohistochemistry examinations should be considered as prognostic factors. Recurrent or metastatic mast cell tumors can show more aggressive patterns regarding primary tumors, wherefore clinical monitoring is very important during disease in these patients.