The standardized assessment of argyrophilic nucleolar organizer regions in meningeal tumors. Tissue markers of cellular proliferation have been recently utilized as prognostic indicators in tumors of the central nervous system. Nucleolar organizer regions represent transcriptionally active sites of ribosomal deoxyribonucleic acid (DNA) and can be identified by a simple argyrophilic technique. The authors describe a standardized approach to the assessment of these argyrophilic nucleolar organizer regions in meningeal tumors. Twenty-five meningiomas were classified histologically into benign, atypical, or malignant groups. In addition, two hemangiopericytomas and one leptomeningeal melanoma were examined. Appropriate sections were silver stained and argyrophilic nucleolar organizer regions were counted in 200 nuclei. The mean argyrophilic nucleolar organizer region count was statistically different (p less than 0.001) between benign tumors (245 +/- 156, 1.23/cell), atypical tumors (497 +/- 135, 2.49/cell), and malignant tumors (921 +/- 59, 4.61/cell). The count for recurrent meningiomas (544 +/- 76) was also statistically different (p less than 0.02) from non-recurrent tumors (329 +/- 183). The standardized assessment of argyrophilic nucleolar organizer regions can be easily performed by any surgical pathology laboratory without specialized equipment and, in meningeal tumors, may be useful as an independent indicator of biological behavior. 