One hundred and twenty-four patients with primary breast cancer were histologically examined to study blood vessel invasion (BVI) using both factor VIII-related antigen (FVIIIRAg) and Elastica van Gieson (El. v. G.) staining.BVI was found in 43.5% of the subjects. The 10-year survival rate in the patients with BVI was 48.1%, while that for those without BVI as favorable as 75.7% (p<0.001). The frequencies of BVI in papillotubular, solidtubular, and scirrhous carcinomas was 23.3%, 49.0% and 56.7% respectively (p<0.03, p<0.01). The 10-year survival rate for papillotubular carcinomas was 91.3% without BVI versus 42.9% with BVI (p<0.006). The frequency of BVI in node-negative patients was 29.7% while that in node-positive patients was 58.3% (p<0.002). Ten-year survival of node-negative patients with BVI was 63.2%, while that of node-negative patients without BVI was 93.3% (p<0.003).Thus BVI appears to be a major prognostic factor in node-negative breast cancers. In addition, use of both FVIIIRAg and El.v.G. staining proved to be an effective method for studying blood vessel invasion.