Clinical usefulness, particularly significance as a prognostic indicator, of tumor angiogenesis, was investigated in primary breast cancer patients. Angiogenesis was evaluated by an immunostaining to factor VIII antigen, which is an endothelial marker. Out of 220 primary breast cancer patients, 54 were included in the high vessel density group with over 100 counts of factor VIII positive cells/x200 microscopic field. The relapse-free survival rate of the high vessel density group was significantly worse than that of the low vessel density group with less than 100 vessel counts (p < 0.01). A multivariate analysis demonstrated that vessel density is an independent prognostic indicator as potent as nodal status. It was concluded that tumor angiogenesis is a new and potent prognostic indicator in primary breast cancer patients.