Experimental and clinical data indicate that tumor progression is associated with angiogenesis and that an increase in microvascular density (MVD) is associated with a poor prognosis, in both solid and hematological malignancies. No data have been published concerning the relationship between angiogenesis and malignancy grade in canine non-Hodgkin's lymphoma (NHL), which is a neoplasm that shares several biological and clinical characteristics with human NHL. In the present study, we evaluated this relationship in a series of 43 cases of canine NHL. The results demonstrate that both MVD and endothelial area (EA) were significantly higher in high-grade compared to low-grade lymphoma and a good statistical correlation was found between MVD and EA. These data indicate that increased angiogenesis paralleled with increased malignancy grade in canine NHL, which represents an interesting tumor model for studying the role of angiogenesis as an interspecies pathway of tumoral malignancy and biological aggressiveness.