The histological sections of 106 patients, who underwent a primary breast cancer operation in the years 1986 and 1987, were inspected for the presence of tumour vascular invasion. With immunohistochemical methods, tumour vessels were tomographed and the paraffin sections were stained with an antibody, which is sensitive to factor VIII-antigen. A tumour embolus was considered present, if a malignant cell was established within a positively stained lumen. The question of possible influence on the pathogenesis of breast cancer was examined in case of vascular invasion in the histological preparation. In 46 of the 106 patients (43.4%), tumour emboli were found. After an average observation period of 59 months, no tumour progression was seen in 60 patients, 46 patients developed a recurrence of the tumour at the check-up date. The presence of vascular invasion correlates with undifferentiated tumour grading. The probability of relapse- free survival after 5 years was 83.9% within the group without vascular invasion, compared with 47.9% (p = 0.0001) within the group with established vascular invasion. In the multivariate analysis of the factors lymphonodular status, grading and vascular invasion, it was found, that the presence of tumour vascular invasion with a relative risk of 3.68 (95% CI 1.93-7.03) was the strongest statistically significant prognostic factor for relapse-free survival (p = 0.0001).