516 cases of supratentorial glioblastoma, treated in the Department of Surgical Neurology, University of Edinburgh from 1950 through 1970, were analysed with a particular attention to the effect of radiotherapy and the factors contributing to better prognosis. The length of postoperative survival was known in 349 cases and it was more than one month in 238 cases. A megavoltage linear accelerator was introduced in 1955 for radiotherapy of glioblastoma and steroids were started to be used almost routinely in 1966 for pre- and postoperative peritumoral cerebral edema. The policies of the treatment of glioblastoma in this series were; 1) to establish the histological diagnosis and 2) to prolong "useful" postoperative survival of the patient. Among the 516 cases, 271 cases (52.5%) were treated by biopsy alone. More radical procedures and/or radiotherapy were indicated only when survival of the patient was expected to be "useful" for himself and his family. "Useful" life was defined as the condition where the patient was conscious and orientated, and would, on the whole, be glad that he was still alive. Patients with disturbed consciousness, profound aphasia, bedridden or mentally disorganized, were regarded as having useless life. Radiotherapy, if indicated, was given by a 4-megavoltage linear accelerator to the whole brain with a total dose of 4500 rads in a period of 4 weeks. In order to evaluate the effect of radiotherapy, 238 cases who survived more than one month postoperatively were selected, because it took at least one month to complete the course of radiotherapy, because most cases with a biopsy alone survived less than one month, and because unexpected early death due to postoperative complications occurred in one month. The average survival for the irradiated 138 cases was 13.8 months, as compared to 5.2 months for the non-irradiated 100 cases. This difference of 8.6 months was highly significant as confirmed statistically by U-test. Although patients with profound aphasia or severe dimentia were not irradiated, aphasia and dimentia would not affect the length of biological survival of the patient. Therefore, the difference of 8.6 months could be considered as the biological effect of radiotherapy. Among the factors considered, young age, early epilepsy and relatively benign histology (astrocytoma, grade 3) appeared to be related to better prognosis. There was no evidence that a macroscopic circumscribed appearance would contribute to better prognosis. In conclusion, radiotherapy should be indicated for the cases whose survival is expected to be "useful", although its effect is limited.