Immunohistochemical analysis of inflammatory cell density and infiltrate subpopulations in 42 meningiomas was performed. Evaluation of infiltrating cell density was carried out by cell counting. Meningothelial and fibroblastic meningiomas contained an average of 3% mononuclear cells; the few lymphocytes were localized in the perivascular spaces. In subtypes with cellular atypies and recurrent tumors, the inflammatory cells increased up to 9%. We found small mononuclear cell clusters in the tumor parenchyma in addition to the perivascular infiltrates. Marked degrees of infiltration were found in anaplastic meningiomas (average 13.5% of total cells). The lymphocytic infiltrates were localized in multilayered perivascular cuffings and intraparenchymal cell clusters. The composition of the infiltrates, i.e., predominantly a mixed staining of cytotoxic/suppressor and helper cell phenotypes, did not vary in the different subtypes. We conclude: (1) that inflammatory infiltration is more frequent and denser in malignant than in benign meningiomas; and (2) that the tumor defense mechanisms in meningiomas are mediated particularly by T cell mediated immunity.