Breast cancer prognosis has previously been linked to the degree of tumour vascularisation. In order to establish additional markers for tumour angiogenesis, we have used monoclonal antibodies against the endothelial Tie receptor tyrosine kinase to study the degree of vascularisation of breast carcinomas and the regulation of Tie expression in the vascular endothelial cells. Antibodies were used for Tie detection and the results were correlated with other prognostic markers. Of four monoclonal antibodies directed against different epitopes of the Tie extracellular domain, two reacted against Tie in unfixed histopathological sections of breast carcinomas. One of these antibodies (clone 7e8) was specific for the endothelial cells whereas the other (clone 10f11) also reacted with basement membranes and occasional carcinoma cells. When Tie expression was studied with the antibody clone 7e8, all 27 carcinomas, two in situ carcinomas, samples of histologically normal breast tissue (n = 16) or normal skin or lymph node tissue (n = 5) showed staining. Microvessel counts were higher in carcinomas (median 14; range 3-27) than in fibrodenomas (median 10; range 5-18) or histologically normal breast tissue (median 7; range 3-15, P = 0.0006). A similar result was obtained using antibodies against the CD31 (PECAM) antigen. Microvessel counts in 7e8 staining were not significantly associated with primary tumour size, axillary nodal status, histological grade or staining for oestrogen receptor, progesterone receptor, Ki-67 proliferation marker or p53 oncoprotein.