COX‐2 overexpression is associated with several hallmarks of carcinogenesis such as proliferation, angiogenesis, invasion and metastasis. Fifty cases of canine mast cell tumours (MCT) were retrospectively evaluated and submitted to immunohistochemistry for COX‐2, CD31, Ki‐67, MAC‐387 and CD3. Furthermore its relationship with clinicopathological variables and overall survival (OS) was analysed. COX‐2 intensity (P = 0.016), but not COX‐2 extension nor score was associated with decreased OS and higher grades of malignancy according to Patnaik (P = 0.002) and Kiupel (P < 0.001) grading systems. Cox‐2 intensity was also associated with higher Ki‐67 scores (P = 0.009), higher mitotic index (P = 0.022) and higher microvascularization density (P = 0.045). No association was observed for COX‐2 intensity and CD3‐T lymphocyte (P = 0.377) and macrophage infiltration (P = 0.261) by …