Cutaneous mast cell tumors (MCTs) are among the most frequent malignant tumors in dogs and Boxer breed dogs have a higher incidence of this disease. Ki67 staining and KIT staining are widely used to predict natural behavior in canine MCT but no previous study has evaluated double staining of these proteins as a prognostic factor. Based on biological behavior predictors in canine MCT, the purpose of this study was to determine the Ki67 proliferative index in KIT positive cells using double stain immunohistochemistry technique. Sixty-nine MCTs from Boxer dogs were selected and a tissue microarray was constructed for the double stained immunohistochemistry. Double positivity (Ki67(+)/KIT(+)) was observed in 20/69 (29%) MCT, with a mean of 9.06 double positive cells per tissue core (range 0.48%-43.97%) and Ki67(-)/KIT(+) animals had a longer survival time than Ki67(+)/KIT(+) animals (p=0.03). ; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) ; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)