Cyclooxygenase-2 (COX-2) is known to be involved in tumour progression and has been suggested as a therapeutic target in many human and animal malignancies. A number of different pathways subjacent to cancer hallmarks are considered to be involved in COX-2-mediated tumour progression, although these are still largely undefined. Our aim is to investigate associations between COX-2 expression and angiogenesis, proliferation and the inflammatory microenvironment in canine melanocytic tumours. Understanding the involvement of COX-2 with cancer hallmarks might enable us to adapt therapeutic strategies for canine melanomas, an aggressive and often lethal malignancy with value in comparative oncology. Immunohistochemical staining of COX-2, Ki-67 (proliferation index), vascular endothelial growth factor (VEGF), factor VIII (microvessel density), CD3 (lymphocytes) and MAC387 (macrophages) was performed in 51 melanocytic tumours (31 malignant melanomas, 20 melanocytomas). Statistical associations between COX-2 and the other parameters detected were analysed. In melanocytic tumours (n=51), both COX-2 labelling extension and intensity showed a statistically significant association with angiogenesis by factor VIII, VEGF, Ki-67, CD3+ T lymphocytes and MAC387. Within malignant melanomas, COX-2 expression has shown significant associations with microvessel density (factor VIII), lymphocyte and macrophage infiltration and, considering all melanocytic tumours, COX-2 was also associated with VEGF intensity and Ki-67 cell proliferation. Our results point to a role for COX-2 in angiogenesis and in the establishment of an inflammatory microenvironment, favourable to melanoma tumour progression. Further mechanistic studies are warranted to dissect molecular pathways in which COX-2 is involved. Present evidence suggests that COX-2 inhibitors might be useful as an adjuvant treatment to hinder canine melanoma progression.