New vessel formation (angiogenesis) plays an important role in the metastasis of cancer cells. To investigate the correlation between tumor angiogenesis and metastatic potential in breast cancer, the microvessel counts of tumor specimens from 81 women with primary infiltrating ductal carcinomas were examined. Histologic parameters (nodal status, tumor size and tumor grade) and hormone receptor status were also analyzed. We found that axillary node metastasis correlated significantly with the microvessel counts per 200x field and with tumor size, but not with age, tumor grade or hormone receptor status. Tumors without axillary node involvement had a lower microvessel count, irrespective of age, tumor size, tumor grade or hormone receptor status. Logistic regression demonstrated that the microvessel count provided the most important estimate of the relative risk of metastasis. These results suggest that, in invasive breast carcinoma, angiogenesis is closely related to metastatic potential.