The histopathologic features of glioblastomas treated with both 1,3 bis (2-chloroethyl)-1-nitrosourea [BCNU] and irradiation after surgery were compared with those of glioblastomas treated with irradiation alone after operation and glioblastomas treated by surgery alone. Tumors treated with BCNU and irradiation showed hypocellular areas that often were associated with a marked numerical increase in giant and monstrous cells. Some irradiated neoplasms had hypocellular areas but these were not as prominent, nor were bizarre giant cells as numerous, as in the BCNU-radiation group tumors. The surgically treated glioblastomas showed only minor changes between surgery and death. The morphologic alterations seen after BCNU-radiation therapy—hypocellularity and an increased number of giant and monstrous cells— are probably indicative of cell division.