Angiogenic homeobox genes regulate the behaviour of endothelial cells (ECs) during angiogenesis, so the aim of this study was to determine whether expression of these genes may be a determinant of malignancy in canine haemangiosarcoma (HSA). Homeobox proteins were evaluated immunohistochemically in tissue samples from canine HSAs (n=78), haemangiomas (HAs; n=30) and samples of granulation tissue (n=8). Active ECs in granulation tissue were positively labelled by antisera specific for HoxA9, HoxB3, HoxD3, HoxB7, Pbx1 and Meis1. Quiescent ECs in granulation tissue did not express HoxD3 and Pbx1. There were significantly more neoplastic cells positively labelled for HoxA9, HoxB3, HoxD3 and Pbx1 in HSA compared with HA. Almost all tumours were positive for HoxB7 and Meis1. HoxB3, HoxD3, Pbx1 and Meis1 proteins were detected in 80-90% of the HSAs, but in <20% of the HAs. Overall, homeobox protein expression in HSA appears to have a phenotype similar to that of active ECs in angiogenesis. The expression of homeobox genes associated with angiogenesis might be associated with the malignant growth of HSA.