Twenty-one malignant cerebral tumors treated with chemotherapy after operation or clinical diagnosis were studied. The brains were cut in coronal slices that were embedded in paraffin. The histological picture of the whole tumor and of the adjacent normal tissue was carefully investigated, and it was compared with the histological features noted on biopsy. The general characteristics of glioblastoma, including wide central necrosis, were present in all of the cases. No specific alterations referable to chemotherapy were found. The increase of monstrous cells and the decrease of mitoses, although observable in some areas, were not of general importance. There was a statistical relationship between the occurrence of reactive astrocytes with bizarre nuclei and repeated chemotherapy or length of time after radiotherapy.