Intracranial tumor classification is paralleled by a grading system that empirically compares tumor entities with "progression stages" of supratentorial gliomas of the adult. This grading system is an integral part of the WHO classification. Glioma progression has originally been defined by descriptive morphology. In this respect, morphological key features of high-grade gliomas (WHO grades III and IV) are microvascular proliferation and the formation of tumor necroses. Glioma progression is now more accurately defined on the molecular genetic level by a stepwise accumulation of oncogene activation and/or tumor suppressor gene inactivation. Angiogenesis occurs during development and progression of glial tumors. Pathological vessels are a hallmark of malignant glioma and it has therefore been suggested that malignant glioma cells are able to induce neovascularization. Despite the exuberant neovascularisation, however, vascular supply may not be sufficient for tumor areas with high cell proliferation, and necroses may develop. Malignant transformation of blood vessel itself is a rare event but may be the underlying mechanism of gliosarcoma development. The recently purified vascular endothelial growth factor (VEGF) is at present the only mitogen known to selectively act on endothelial cells. Growing evidence suggests that VEGF is the key regulator of developmental and pathological angiogenesis. In vivo, VEGF mRNA is upregulated in a subpopulation of malignant glioma cells adjacent to necroses. Since VEGF is hypoxia-inducible, hypoxia may be an important regulator of VEGF mRNA expression and tumor angiogenesis in vivo. Two tyrosine kinase receptors for VEGF are expressed in vessels which invade the tumor, suggesting that tumor angiogenesis is regulated by a paracrine mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)