The number of nucleolar organizer regions in vascular cells and neoplastic cells of human gliomas was counted by a combined staining technique: one-step silver colloid method for nucleolar organizer region-associated argyrophilic protein (AgNOR) and periodic acid-Schiff staining for basement membrane of vascular components. The number of AgNORs (mean +/- SD) in the vascular and neoplastic cells of various tumors tested was as follows: benign astrocytoma (Grade 2, n = 4), 1.52 +/- 0.07 and 1.80 +/- 0.13, respectively; anaplastic astrocytoma (Grade 3, n = 7), 1.98 +/- 0.23 and 2.87 +/- 0.50; and glioblastoma multiforme (Grade 4, n = 11), 2.05 +/- 0.29 and 3.13 +/- 1.13. AgNOR scores in vascular cells of benign astrocytomas, anaplastic astrocytomas, and glioblastomas were significantly higher than those of vascular cells in normal brain tissue without neoplastic alteration (1.26 +/- 0.05; n = 3; P less than 0.01, P less than 0.001, and P less than 0.001, respectively). Moreover, the AgNOR scores of vascular cells in high-grade gliomas (Grades 3 and 4) were significantly higher than those in low-grade gliomas (Grade 2) (P less than 0.01). These results indicate that the proliferative activity of both vascular and neoplastic cells in gliomas increased as histological malignancy advanced, and that the quantification of AgNORs was useful in evaluating proliferative activity in vascular cells as well as in assessing the malignancy of neoplastic tissues.