The pathological effects of irradiation and chemotherapy have been studied in 9 autopsy cases of malignant and low grade gliomas. The brains have been examined by means of the complete study technique. Many histological features have been related to surgery, grading of histological classification of gliomas, irradiation and chemotherapy. Following irradiation and chemotherapy, in addition to increased necrosis and vascular response, a variety of characteristics changes were observed in cell and nuclear morphology with prominent formation of monstrous cells in all of 5 malignant gliomas treated with nitrosourea. These monstrous cells had irregular and hyperchromatic multiple nuclei and showed cytoplasmic degeneration. These cells which had not direct relationship to vessels distributed both in the periphery of tumor or necrosis and in the white matter remote from the main tumor. These changes were more pronounced in autopsy than in biopsy. The features showed here indicate that the monstrous cells may appear due to the result of inhibition of tumor cell division at the late mitotic phase after irradiation and chemotherapy.