In this study the effect of suramin on tumor growth, vascularity and oxygenation of a human glioma xenografted in the nude mouse was examined. Vascular parameters and oxygenation status of the xenografts were determined immunohistochemically in frozen sections of the tumors, using the hypoxia marker pimonidazole-hydrochloride to detect hypoxic areas. Tumor vessels in these sections were stained by an endothelial cell marker and perfusion of vessels was visualized by administration of the perfusion marker Hoechst 333342 before harvesting the tumors. The vascular parameters were quantified with an image analysis system. The results show that tumor growth was reduced considerably after suramin treatment. This growth suppression was accompanied by marked changes in vascular architecture. Although the total vascular area and perfused fraction of tumor vessels remained unchanged after suramin treatment, vascular density increased, indicating that more but smaller vessel structures had developed during therapy. These vessel structures were also more homogeneously spread over the tumor area. Control tumors showed extensive areas of hypoxia while in treated tumors hypoxic areas had mostly disappeared. This effect was probably due to the higher density of homogeneously distributed perfused vessel structures in the treated tumors, contributing to an increased oxygenation of the tumor. These observations suggest that suramin therapy can result in marked changes not only in tumor vascularity but also in tumor oxygenation status which may have important consequences for sensitivity of these tumors to other therapies such as radiation treatment.