The assessment of a tumour's angiogenic potential, by measuring the microvessel density in histological sections, assumes that a 4-microm section is representative of whole tumour vascularity. This study has examined this assumption by comparing the vessel density found radiologically, after injecting specimens with contrast, with that found immunohistochemically. Twenty-one breast angiograms were performed following mastectomy for carcinoma and graded 1-3 for vessel density. Sections (4 microm) from these carcinomas were labelled for endothelial cells using anti-CD34, and the vessel counts were compared with the radiological grades. A significant correlation was found between the densities (P < 0.003, Kruskal-Wallis one-way ANOVA). We therefore conclude that the microvessel density measured in histological sections is representative of whole tumour vascularity.