Apoptosis is a physiological process wherein the cell initiates a sequence of events culminating in the fragmentation of its DNA, nuclear collapse, and finally disintegration of the cell into small, membrane-bound apoptotic bodies. Expression of Fas (APO-1, CD95) Receptor (FasR) and programmed or active cell (PCD) death was studied in childhood astrocytomas (ASTRs) with varying stages of malignancy, including pilocytic ASTR, low grade ASTR, anaplastic ASTR, and glioblastoma multiforme (GBM). The great majority of childhood glial tumors, particularly ASTRs express FasR whereas normal cells in the central nervous system (CNS) do not. FasR represents a transmembrane glycoprotein which belongs to the nerve growth factor/tumor necrosis factor (NGF/TNF) receptor superfamily. Apoptosis within ASTRs is triggered by the binding of FasR to its natural ligand (FasL) or by cross-linking with antibodies developed against FasR. Presence of FasL was also detected in childhood glial tumors. The expression of both FasR and FasL was also observed within the same ASTRs. Therefore, spontaneous, IP regulatory, intratumoral apoptotic cell death (autocrine suicide) is possible in childhood glial tumors. During a systematic, immunocytochemical screening of 42 childhood ASTRs tissues divided according to WHO classification: 6 WHO grade I or pilocytic ASTRs; 14 WHO grade II or low grade ASTRs; 16 WHO grade III or anaplastic ASTRs and 6 WHO grade IV or glioblastoma multiforme (GBM), we detected strong expression (intensity of staining: "A"--the highest possible; number of stained cells: +2 to +4, between 20% to 90%) of FasR, employing 4 microns thick, formalin fixed, paraffin-wax embedded tissue slides. FasR was present on 70% to 90% of tumor cells in pilocytic ASTRs, in 50% to 60% of the tumor cells in low grade ASTRs, in between 30% and 40% of the tumor cells in anaplastic ASTRs, and in between 20% to 35% of GBM cells. The panel of normal tissues employed as positive and negative tissue controls demonstrated presence of FasR in the prenatal thymus, mature tonsils and colonic epithelium. The use of a sensitive, indirect, six step immunoperoxidase or alkaline phosphatase conjugated streptavidin-biotin antigen detection technique provided excellent immunocytochemical results. A broad spectrum of neoplastic cells have been identified to express FasR: 1) carcinomas of epithelial origin, such as breast (ductal invasive, lobular invasive, mucinous), renal cell, gastric, colorectal, endometrial, prostate, pancreas, hepatocellular and large cell and squamous cell lung carcinomas: 2) non-epithelial neoplasms such as B cell mediastinal B cell and nodal non-Hodgkin's lymphomas large granular lymphocytic leukemia of T or NK cell origin malignant fibrous histiocytoma, malignant mesothelioma, leiomyosarcoma, epitheloid sarcoma and alveolar soft part sarcoma, as well as melanomas. Flow cytometry studies have also detected FasR expression on cells of adult T cell, and hairy cell leukemias, as well as in chronic B cell lymphocytic leukemia (BCLL). The coexpression of both FasR and FasL on several malignant cell types may represent an effective mechanism of tumor escape from the cellular immunological response of the host. It has been well established that brain tumors and melanomas produce their autocrine FasL, and even become capable of switching the signal transduction associated with FasL-FasR coupling from the PCD pathway to a tumor growth, proliferative pathway. It seems that the therapeutical use of FasR-FasL (main apoptotic pathway) may represent a new and exciting type of immunotherapy in the treatment of primary childhood glial tumors.