Experimental and clinical data indicate that tumor progression and malignancy are associated with increased angiogenesis and higher Ki-67 proliferation rate. Furthermore, increased angiogenesis and higher Ki-67 proliferation rate are associated with a poor prognosis, in both solid and hematological malignancies. However, no data have been published concerning the relationship between angiogenesis and Ki-67 proliferation rate in canine non-Hodgkin's lymphoma (NHL), a neoplasm that shares several biological and clinical characteristics with human NHL. This study has evaluated the relationship between angiogenesis and Ki-67 proliferation rate in a series of 58 canine NHL. Results showed that microvascular density (MVD), endothelial area (EA) and Ki-67 (MIB-1) are significantly correlated and that all the above indexes paralleled with the malignancy degree of NHL. These data suggest a biological link between angiogenesis and Ki-67 proliferation rate in canine NHL, which represents an interesting model to study the role of angiogenesis and proliferative activity as inter-species pathways of tumoral malignancy and biological aggressiveness.