Tumour angiogenesis is a powerful prognostic indicator in breast cancer. Shedding of tumour cells into the bloodstream is essential for haematogenous metastasis. The relation between cell shedding and angiogenesis in human cancer is not known. We studied vascular density and cell shedding in 16 women undergoing breast cancer surgery. Circulating cells were found in one patient before, in six during, and none after operation. Vascular density was related to detection of circulating tumour cells during surgery (correlation coefficient for first intraoperative sampling = 0.56, p = 0.024, 95% CI 0.08 to 0.83). Tumour cell shedding during surgery is related to vascular density; the relevance of this relation to the prognostic significance of vascular density in breast cancer merits further study.