Although florid microvascular proliferation (MVP) in glioblastoma multiforme (GBM) has long been considered as proliferation of endothelial cells (EC), recent immuno-light microscopic studies demonstrated many alpha-smooth muscle actin (alpha-sm actin)-positive cells in this MVP, suggesting a major contribution of pericytes and/or vascular smooth muscle cells (VSMC). Under certain culture conditions, however, alpha-sm actin expression has also been described in EC. In order to further investigate to what extent pericytes/VSMC participate in MVP in GBM, we performed an immunohistochemical study at both the light and electron microscopic levels with anti-alpha-sm actin, with an antibody against EC (EN-4) and with an antibody recently described to react with "activated" pericytes in conditions with neovascularization (anti-high molecular weight-melanoma associated antigen). In this detailed study of MVP in GBM, two distinct cell types could be recognized on the basis of a consistent ultrastructural localization and immunophenotype: EC and pericytes/VSMC; no transitional forms were found between these two cell types. The contribution of pericytes/VSMC to MVP in GBM was extensive and already present in many delicate tumor capillaries, suggesting not only an essential but also an early role of these cells in this type of tumor angiogenesis.