Twenty-six cases are reported of gliosarcoma (GS) retrieved from a series of 1479 glioblastomas (GBM) that were part of five consecutive, randomized Phase II or III malignant glioma protocols initiated by the Radiation Therapy Oncology Group between 1974 and 1983. The clinicopathologic features of these 26 cases, including actuarial survival times, were compared with the remaining 1453 GBM. The minimal qualitative and quantitative histologic criteria required to diagnose GS are presented. In most cases the sarcomatous component was a malignant fibrous histiocytoma; a minority were fibrosarcoma. No significant differences between GS and GBM were found with regard to age, sex, pretreatment Karnofsky performance status, tumor location, size, median survival (8.3 and 9.6 months, respectively), and actuarial survival. None of the treatment regimens, which included various combinations of radiation therapy and chemotherapy, improved the survival of GS over GBM. Selective involvement of the temporal lobe by GS was not found, and the frequency of GS was determined to be only 1.8% of all GBM.