Animal models suggest a role for new vessel formation (angiogenesis) in tumours with metastatic potential, and there is some evidence that this is true for human tumours. What is needed is a sensitive and specific label for endothelial cells, and one candidate would be a monoclonal antibody to platelet/endothelial cell adhesion molecule (PECAM). We have counted microvessels in 103 primary breast cancers using the JC70 antibody to PECAM (or CD31). We compared our findings with various pathological indicators (lymph node status and tumour grade, size, and type and markers (oestrogen receptor, and c-erbB-2 expression and detection of mutant p53). Tumours showed significantly higher vascularisation than normal breast tissue and the number of blood vessels/mm2 was significantly associated with node metastasis. Only 2 out of 50 tumours with 99 vessel/mm2 or less were node positive whereas 31 out of 39 tumours with counts above 140/mm2 were positive (p < 0.0001). Tumour size and grade also correlated with node metastasis and vascularisation also increased with the size of the primary and with poor differentiation. However, within each subgroup of size or differentiation tumours without node involvement had much lower vascular counts, and multivariate analysis showed that vascular count alone explains the association of size and grade with node metastasis. Other markers, conventional or novel, did not correlate with vascularisation. Even with the short follow-up in this series, vascular counts correlated with early death. These results suggest that angiogenesis is closely linked to metastasis, that it is acquired at a critical density of vessels, and that this process occurs as tumours enlarge or become more poorly differentiated. Counting of newly formed microvessels stained with endothelium-specific antibodies may prove to be a useful tool in the early detection of metastatic potential and in the selection of patients for whom anti-angiogenesis drugs might be beneficial.