Neovascularization is a key step in the transformation of normal breast tissue into a cancerous phenotype. We investigated the angiogenic activity of 58 cases of histologically confirmed benign breast lesions quantitating angiogenic activity by factor VIII highlighting of endothelial cells. The mean microvessel count (MVC) for all patients was 51 per 0.19 mm2 in the area of highest angiogenic activity at 400X magnification (range 21-113 per 0.19 mm). MVC in benign tissues were similar to those found previously in cancer tissues. These data suggest that the 'angiogenic switch' occurs early in breast transformation and that a role may exist for the use of MVC in the identification of persons at heightened risk for subsequent development of breast cancer.