The process of angiogenesis is a prereguisite for normal cell growth and differentiation. Neovascularization might also be important for tumor growth in the host tissue and for the establishment of growth in distant target tissue. Many studies have suggested that the presence of neovascularization (microvessel density) can be used as a biologic marker" for the prediction of subsequent metastasis or mortality in breast cancer patients. To evaluate the relationship between microvesse1 density and other known prognostic indicators in breast carinoma, we performed immunoperoxidase staining using CD31 (IC70, Dako) on 80 cases of formalin fixed human breast cancer tissue, and counted microvessel density(MVD) per xl00 field in the most active areas of neovascularization. Also mamma- ry tumors were experimentally induced by feeding of 9,10-dimethyl-1,2-benzan-thracene (DMBA) to female Syrague-Dawley rats, and then the angiogenic activity was observed during neoplastic transformation by immunoperoxidase staining for factor VII:-related antigen. The mean value of MVD was 78.70+23.95 in the node negative group, and 91.2326.18 in the node positive group. Thus there was a significant (p=0.0285) association of MVD with axillary nodal status. However, there was no association between MVD and other prognos- tic factors, i.e, estrogen and progesterone receptors, age, tumor size, and nuclear grade. Multivariate analysis by Cox's proportional hazard model disclosed that MVD, sge of patients, axillary nodal status and nuclear grade could influence patients prognosis. Breast masses developed in 68% (25/37) of DMBA treated rats, and revealed fibroadenomatous hyperplasia, adenomatous hyperplasia (hyperplastic alveolar nodule) and adenocarcinoma with increased neovascularization depending on size and duration. This study concludes that the breast masses in DMBA treated rats can be used as an experi- mental animal model of neoangiogenesis in breast cancer.