Assessment of angiogenesis has been reported to be an independent prognostic factor in breast cancer, while other studies have been negative. This study prospectively investigates the degree of intratumoral microvessel heterogeneity and the possible influence on the results. From 21 invasive breast cancers six 4 micro sections were cut. Sections (n=126) were stained immunohistochemically with a CD31 monoclonal antibody (JC70). In each section, three areas with the most intense neovascularisation (hot spots) were identified and the microvessel density (MVD) was obtained by counting vessels at 200x magnification. The variation between sections contributed more to the total variance than variation between different tumours: 45.0 and 37.3%, respectively, according to a nested ANOVA analysis. Paired comparisons of two sections at a time from the same tumour showed a concordance in 59.0% (95% Confidence Interval (CI): (55.3-62.8)) with reference to a tentative cut-off level. Our study demonstrates that assessment of MVD in hot spots is questionable to measure angiogenesis due to the considerable intratumoral heterogeneity.