Angiogenesis is essential for tumour growth and metastasis. Although vascular density as a measure of angiogenesis is an important prognostic factor in invasive breast carcinoma, the mechanism of a switch to an angiogenic phenotype in ductal in situ breast carcinomas (DCIS) has yet to be identified. Nevertheless, two distinct vascular patterns have been reported in DCIS: a diffuse increase of stromal vascularity and a dense rim of microvessels close to the basement membrane of involved ducts. This suggests that tumour angiogenesis in invasive breast cancer arises from two different angiogenic pathways. Platelet-derived endothelial cell growth factor, now known to be thymidine phosphorylase (TP), is a candidate for initiating one of these pathways, since it is important in remodelling the existing vasculature through its chemotactic non-mitogenic properties and is expressed early in breast cancer development. The expression of TP was therefore examined in 75 formalin-fixed, paraffin-embedded specimens of DCIS by immunohistochemistry, using the monoclonal antibody PGF44c to detect TP. The results were correlated with blood vessel staining by polyclonal antibodies to von Willebrand factor (Factor VIII-related antigen, FVIIIrAg) and other clinicopathological variables. TP expression was nuclear and/or cytoplasmic and was observed in all subtypes of DCIS. High TP expression was demonstrated in 36 per cent (27/75) of tumours. This was not limited to the neoplastic cells, but was also present in stroma, endothelium, and tumour-associated macrophages. There was no correlation between high TP and DCIS subtype (P > 0.05). There was a significant correlation between TP expression and the presence of a dense vascular rim (P = 0.042; chi 2 = 4.1), but not with an increase in stromal vascularity (P = 0.800; chi 2 = 0.1). There was no significant correlation between tumour TP expression and relapse-free survival (P = 0.662; chi 2 = 0.2). These findings suggest that remodelling of the pre-existing vascular network induced by TP is important in generating a dense rim of microvessels around DCIS.