The pattern of inflammatory infiltration was studied in the frontal brain biopsies of 28 cases with subacute sclerosing panencephalitis (SSPE) by immunohistochemistry. Lymphocytic infiltration and gliosis were common pathologic findings. CD4+ T lymphocytes were often observed in perivascular areas and CD8+ lymphocytes in the parenchyma. B lymphocytes were located in large perivascular cuffs associated with longer and slower disease. Major histocompatibility complex antigens, interferon-gamma, and tumor necrosis factor-alpha (TNF-alpha) were expressed in endothelial and glial cells. The inflammatory lesions in subacute sclerosing panencephalitis consist of various cell subtypes and cytokines localized in particular areas of the brain tissue and show certain associations with clinical course.