Blood vessel density is a prognostic indicator of multiple tumor types. Recently, it has been established that tumor-associated blood vessels express elevated levels of integrin alpha(v)beta3. In fact, there is evidence that integrin alpha(v)beta3 identifies the most proliferative endothelial cells within human breast carcinomas. Therefore, we evaluated breast cancer tissue in terms of both blood vessel density and alpha(v)beta3 expression. We found that the antibody LM609 to integrin alpha(v)beta3 preferentially stains the blood vessels of small caliber. Furthermore, comparative studies between LM609 and anti-CD31 antibodies on normal breast indicate that very low and weak expression of integrin alpha(v)beta3 was found on vessels within normal tissue, whereas CD31 antigen was expressed in almost all vasculature. Indeed, expression of integrin alpha(v)beta3 was significantly higher in tumors of patients with metastasis than in those without metastasis. In a series of 197 consecutive patients with invasive breast cancer and long follow-up, vascular expression of integrin alpha(v)beta3 in tumor vascular "hot spots" was found to be the most significant prognostic factor predictive of relapse-free survival in both node-negative and node-positive patients. These findings support the contention that angiogenesis plays a critical role in breast cancer progression and suggest that integrin alpha(v)beta3 is an endothelial cell marker with significant prognostic value and potential usefulness as a target for specific antiangiogenic therapy.